QtCanvas

Adds QWidget subclasses that make it more natural to work with vispy from PyQt applications. The widgets `QtCanvas` and `QtSceneCanvas` each wrap methods from their `Canvas` and automatically embed the native QGLWidget.

Depends on #529
Hmm maybe you need these so that the Qt `Application` is created...? I wonder if we could move these to `backends._qt` and import them in a more convenient place for users, or otherwise make it so you don't need these lines here.