The order and way include macros were structured was chaotic, with no
real common thread between files. Try to tidy up the mess with some
common scheme, to make things look less messy.
We must emit ::dnd-leave to pair the ::dnd-enter that shall be
emitted whenever the plugin grab begins, otherwise we leave
listeners unable to clean up if the plugin begins and ends a
grab while there is an ongoing DnD operation.
https://bugzilla.gnome.org/show_bug.cgi?id=784545
No XDnD events which notify DnD status change comes in Wayland. To emulate XDnD
behavior, MetaDnd checks whether there is a grab or not when the modal window
starts showing. When there is a grab, it processes the raw events from
compositor, and emits DnD signals for plugin.
https://bugzilla.gnome.org/show_bug.cgi?id=765003
Implement MetaDnd for emitting DnD signals to plugins such as gnome-shell. The
xdnd handling code comes from gnome-shell, and it is hidden behind MetaDnd now.
https://bugzilla.gnome.org/show_bug.cgi?id=765003
