Cut down on boilerplate by using the (no longer that) new helper
macros. We don't care about breaking ABI in private libraries, so
use G_DECLARE_FINAL_TYPE even where the class struct used to be
exposed in the header, except for types we inherit from ourselves
(obviously) or where the class exposes any vfuncs (where changes
could affect inheritance in extensions).
Use a new ShellGLSLQuad actor class to build a RadialEffect that can be
enabled on Lightboxes to achieve a radial effect similar to the overview
one. Then enable it for modal dialogs.
https://bugzilla.gnome.org/show_bug.cgi?id=669798