clutter-event: Add scroll source enum and axis scroll flags

Those can be used to implement different scrolling behaviors.
The fields have been added to ClutterScrollEvent itself. According
to pahole, this makes the struct as big as ClutterButtonEvent and
ClutterTouchEvent, so already at the limit of the ClutterEvent
union.

https://bugzilla.gnome.org/show_bug.cgi?id=757026