Simplify test-cogl-primitives

Simplify test-cogl-primitives

This removes a lot of code from test-cogl-primitives to make it easier
to follow. The TestCoglBox custom actor has gone and instead a blank
ClutterGroup is created with a paint signal handler.

Instead of rendering constantly and updating when a GTimer elapses a
second, a ClutterTimeline is used with 1 fps and a new redraw is
queued every frame.

The custom main loop is replaced with a regular call to clutter_main.
This fixes the close button of the stage window so you can quit
without having to press Ctrl+C.