Add annotations to all exported functions in GTK+ headers.

gcc has optimizations for include guards that only work
if they are outermost in the the header.
https://bugzilla.gnome.org/show_bug.cgi?id=689810

https://bugzilla.gnome.org/show_bug.cgi?id=640999

Seems commit 7b42d4fe makes
GtkSpinner a direct subclass of GtkWidget but forgets to
update the header file to include gtkwidget.h instead and
declare the instance and class structures properly (assuming
this was just a missed file in the commit).

Notify spinner::active only if it really changed an other cosmetic fixes

Add GtkSpinner activity throbber, as well as a cell renderer.