[tui] Add EditorTab widget.

+ This adds support for tabbed editors wrapped by EditorTab widgets.
+ The Explorer widget now opens new EditorTabs when a file is selected
  with Enter.
+ The same file may not be opened multiple times.
+ Tabs can be switched with ALT+h or ALT+l (or ALT+ arrow keys)
+ Tabs cannot yet be closed :)

Fixes #9

src/tui/editor_tab.rs

@@ -0,0 +1,151 @@
+use crate::tui::component::{Action, Component};
+use crate::tui::editor::Editor;
+use anyhow::{Context, Result};
+use log::trace;
+use ratatui::buffer::Buffer;
+use ratatui::crossterm::event::{Event, KeyCode, KeyEvent, KeyModifiers};
+use ratatui::layout::Rect;
+use ratatui::prelude::{Color, Style};
+use ratatui::widgets::{Block, Borders, Padding, Tabs, Widget};
+use std::collections::HashMap;
+
+// Render the tabs with keys as titles
+// Tab keys can be file names.
+// Render the editor using the key as a reference for lookup
+pub struct EditorTab {
+    pub(crate) editors: HashMap<String, Editor>,
+    tab_order: Vec<String>,
+    current_editor: usize,
+}
+
+impl EditorTab {
+    fn id() -> &'static str {
+        "EditorTab"
+    }
+
+    pub fn new(path: &std::path::PathBuf) -> Self {
+        trace!(target:Self::id(), "Building EditorTab with path '{path:?}'");
+        let tab_order = vec![path.to_string_lossy().to_string()];
+        Self {
+            editors: HashMap::from([(tab_order.first().unwrap().to_owned(), Editor::new())]),
+            tab_order,
+            current_editor: 0,
+        }
+    }
+
+    pub fn next_editor(&mut self) {
+        self.current_editor = (self.current_editor + 1) % self.tab_order.len();
+    }
+
+    pub fn prev_editor(&mut self) {
+        self.current_editor = self
+            .current_editor
+            .checked_sub(1)
+            .unwrap_or(self.tab_order.len() - 1);
+    }
+
+    pub fn current_editor(&self) -> Option<&Editor> {
+        self.editors.get(&self.tab_order[self.current_editor])
+    }
+
+    pub fn current_editor_mut(&mut self) -> Option<&mut Editor> {
+        self.editors.get_mut(&self.tab_order[self.current_editor])
+    }
+
+    pub fn open_tab(&mut self, path: &std::path::PathBuf) -> Result<()> {
+        if self
+            .editors
+            .contains_key(&path.to_string_lossy().to_string())
+        {
+            return Ok(());
+        }
+
+        let path_str = path.to_string_lossy().to_string();
+        self.tab_order.push(path_str.clone());
+        let mut editor = Editor::new();
+        editor.set_contents(path).context("Failed to open tab")?;
+        self.editors.insert(path_str, editor);
+        self.current_editor = self.tab_order.len() - 1;
+        Ok(())
+    }
+
+    pub fn render(&mut self, tabs_area: Rect, editor_area: Rect, buf: &mut Buffer) {
+        // TODO: Only file name is displayed in tab title, so files with the same name in different
+        //    directories will appear confusing.
+        let tab_titles = self.tab_order.iter().map(|t| {
+            std::path::PathBuf::from(t)
+                .file_name()
+                .map(|f| f.to_string_lossy().to_string())
+                .unwrap_or("Unknown".to_string())
+        });
+        Tabs::new(tab_titles)
+            .select(self.current_editor)
+            .divider("|")
+            .block(
+                Block::default()
+                    .borders(Borders::NONE)
+                    .padding(Padding::new(0, 0, 0, 0)),
+            )
+            .highlight_style(Style::default().fg(Color::LightRed))
+            .render(tabs_area, buf);
+        Widget::render(self, editor_area, buf);
+    }
+}
+
+impl Widget for &mut EditorTab {
+    fn render(self, area: Rect, buf: &mut Buffer)
+    where
+        Self: Sized,
+    {
+        if let Some(editor) = self.current_editor_mut() {
+            editor.render(area, buf);
+        }
+    }
+}
+
+impl Component for EditorTab {
+    fn handle_event(&mut self, event: Event) -> Result<Action> {
+        if let Some(key) = event.as_key_event() {
+            let action = self.handle_key_events(key)?;
+            match action {
+                Action::Quit | Action::Handled => return Ok(action),
+                _ => {}
+            }
+        }
+        self.current_editor_mut()
+            .context("Failed to get current editor")?
+            .handle_event(event)
+    }
+
+    fn handle_key_events(&mut self, key: KeyEvent) -> Result<Action> {
+        match key {
+            KeyEvent {
+                code: KeyCode::Char('h'),
+                modifiers: KeyModifiers::ALT,
+                ..
+            }
+            | KeyEvent {
+                code: KeyCode::Left,
+                modifiers: KeyModifiers::ALT,
+                ..
+            } => {
+                self.prev_editor();
+                Ok(Action::Handled)
+            }
+            KeyEvent {
+                code: KeyCode::Char('l'),
+                modifiers: KeyModifiers::ALT,
+                ..
+            }
+            | KeyEvent {
+                code: KeyCode::Right,
+                modifiers: KeyModifiers::ALT,
+                ..
+            } => {
+                self.next_editor();
+                Ok(Action::Handled)
+            }
+            _ => Ok(Action::Noop),
+        }
+    }
+}