GtkModelButton: specify the proper default values

Caught by the defaultvalue test: ::text starts out as "", and
::iconic should be FALSE by default.
GtkGLArea: avoid extraneous property notifications
notify test: Exclude a few new properties

The button properties don't make sense on GtkModelButton (indicating
that GtkModelButton should really not be a GtkButton subclass).
Update expected results
GtkModelButton: Add docs
Add missing api to the docs
3.15.2
Bump version