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
> Well if I call from .backends._qt import QtGui before a backend is selected,

You should not do that, there is a private module there :) 

Is your point that you want to select a Qt library, but don't care whether that's PySide or PyQt4? This should work btw (though not the prettiest solution):

    a = app.use_app()
    QtGui = a.backend_module.QtGui