Bug 659602 - Provide an abstraction for the platform's use of modifier keys

Bug 659602 - Provide an abstraction for the platform's use of modifier keys

Add enum GdkModifierIntent which identifies use cases for modifier masks
and GdkKeyMap::get_modifier_mask(). Add a default implementation which returns
what is currently hardcoded all over GTK+, and an implementation in the
quartz backend. Also add gtk_widget_get_modifier_mask() which simplifies
things by doing widget->display->keymap->get_modifier_mask().