Simplify some drag dest internals

Simplify some drag dest internals

Change things around to make the default handler
of GtkDropTarget::drag-motion implement the default
handling. Always emit ::drag-motion and ::drag-leave,
and always arm the drop target.

This makes the GTK_DEST_DEFAULT_MOTION flag and
the ::track-motion property unnecessary.

Drop targets that are purely after track-motion
handling for ui switching purposes, such as
GtkStackSwitcher, will have to undo the automatic
arming in a notify::armed handler.