ui.backend.gtk, use a separate widget for drawing opengl. Fixes #1487

ui.backend.gtk, use a separate widget for drawing opengl. Fixes #1487

Apparently you can't use opengl calls in a configure event signaled from
the window. If you do, on ubuntu 14.04 with unity, each and every frame
will be drawn. Since they are drawn slower than the event are emitted,
it continues to redraw after the resize is done, 'replaying' the resize
motion.
opengl calls from a configure event on the single child of the
window are ok.
Also, now that the window is not used to draw opengl, it doesn't produce
expose events, instead the drawable child must be used.

This also makes moving the window much smoother.