settings: Move setting property registration in gtksettings.c

settings: Move setting property registration in gtksettings.c

Some GtkSettings property are registered by other classes. This leads
to the "interesting" issue that setting GtkSettings:gtk-button-images
requires that the GtkButton class is referenced first - or that a
GtkButton is created.

https://bugzilla.gnome.org/show_bug.cgi?id=632538