gl-glxgears: Fix missing MAP event bug.

gl-glxgears: Fix missing MAP event bug.

The MAP event was sometimes arriving before its handler was assigned,
leaving the animation thread to wait forever for the widget to be
mapped.  Fix this by moving the handler assignments to the
initialize-instance method.

The more common way to initialize a widget is in its fix-widget-
realize-callback.  Rename the "mapped?" slot "realized?".  Add an
unrealize-callback method to clear the "realized?" slot, just for
completeness.

Use (fresh-line) and "; glxgears: " consistently before the
asynchronous, running notes.

Also add some missing `free's to the primitive version: glxgears.scm.