GL: Follow naming conventions

All the GDK type defines are GDK_TYPE_..., so follow this
pattern for the GLContext subclasses as well.
Skip GdkX11GLContext in some object tests

Since it can't be constructed without specifying a GdkWindow,
which we don't have here.
Add a 3.16 index

We have new GDK api already.
Add new papersize api to the docs
Add all sidebar API to the docs
Fix docs for gtk_css_provider_load_from_resource
Fix docs for gtk_settings_set_property_value
Document gtk_printer_option_set_get_groups
Document gtk_sidebar_get_stack
GtkSidebar: Make docs show up
Add GtkGLArea to the widget gallery
Add GtkSidebar to the widget gallery
3.15.0
Drop a reference to gtkthemingengineprivate.h

That file does not exist.