Factor out more resources.

+ Use QOpenGLShader to get compiled shader code in TreeView.
+ Remove last remaining Qt resources dependency in libqtk.
+ shaders.h in libqtk to define default GLSL shader programs.

resources/skybox/skybox.frag

@@ -1,9 +0,0 @@
-#version 330
-uniform samplerCube uTexture;
-
-varying vec3 vTexCoord;
-
-void main()
-{
-  gl_FragColor = texture(uTexture, vTexCoord);
-}

resources/skybox/skybox.vert

@@ -1,15 +0,0 @@
-#version 330
-layout(location = 0) in vec3 aPosition;
-
-out vec3 vTexCoord;
-
-uniform mat4 uProjectionMatrix;
-uniform mat4 uViewMatrix;
-
-void main()
-{
-  // Strip translation column from camera's 4x4 matrix
-  mat4 view = mat4(mat3(uViewMatrix));
-  gl_Position = uProjectionMatrix * view * vec4(aPosition, 1.0);
-  vTexCoord = aPosition;
-}