gtkentry: Use gestures for entry icons

gtkentry: Use gestures for entry icons

Instead of doing all handling manually in the ::event vfunc,
set up drag/multipress gestures on icon images, and implement
emission of ::icon-press/release and DnD there.

As a side effect, the GdkEvent field in ::icon-press/release
signals has been dropped. Callers that might be interested on it
may still use gtk_get_current_event*().