Remove QtkWidget dependency on MainWindow.

src/app/qtkmainwindow.cpp

@@ -25,6 +25,7 @@ MainWindow::MainWindow(QWidget * parent) : QMainWindow(parent)
   // Initialize static container for all active QtkWidgets
   auto qtkWidgets = findChildren<Qtk::QtkWidget *>();
   for (auto & qtkWidget : qtkWidgets) {
+    qtkWidget->setMainWindow(this);
     // NOTE: Set a temporary scene for the widget to use for initialization.
     // This should be replaced by loading a scene, or creating a new (unsaved)
     // scene when Qtk is opened.