event: Add convenience functions for Shift and Ctrl modifiers

Instead of going through clutter_event_get_state() and checking if the
modifier mask is set, we can provide simple convenience functions to do
it for us.

doc/reference/clutter/clutter-sections.txt

@@ -1088,6 +1088,8 @@ clutter_event_get_event_sequence
 clutter_event_get_angle
 clutter_event_get_distance
 clutter_event_get_position
+clutter_event_has_control_modifier
+clutter_event_has_shift_modifier
 
 <SUBSECTION>
 clutter_event_get