libgimpwidgets: New API to check system animation status

Ports the animation code started in e13cc635
to an independent gimp_widget_animation_enabled()
function. This allows plug-in authors to
also conditionally turn off animations if
the user's system settings say to do so.

The function is applied to the About
Dialogue animation as well as two Easter
Egg animations:
* Wilber's eyes blinking after 23 minutes
on an empty canvas
* Wilber's eyes following the mouse after
a certain sequence of tools is clicked

libgimpwidgets/gimpwidgetsutils.h

@@ -70,6 +70,8 @@ const Babl         * gimp_widget_get_render_space    (GtkWidget       *widget,
 void                 gimp_widget_set_native_handle   (GtkWidget        *widget,
                                                       GBytes          **handle);
 
+gboolean             gimp_widget_animation_enabled   (void);
+
 /* Internal use */
 
 G_GNUC_INTERNAL void _gimp_widget_get_profiles       (GtkWidget         *widget,