This commit brings a big change to the DND and copy/paste interfaces.
Most importantly the functionality is now independent of wl_shell.
The wl_shell interface is intended for desktop style UI interaction and
an optional and experimental interface.
The new interface also allows receiving the DND data multiple times or
multiple times during the drag, and the mechanism for offering and receiving
data is now shared between DND and selections.
Some events, such as the display.delete_id, aren't very urgent and we
would like to not always send them immdiately and cause an unnecessary
context switch. The wl_resource_queue_event() function will place the
event in the connection output buffer but not request the main loop to
poll for writable. The effect is that the event will just sit in the
output buffer until a more important event comes around and requires
flushing.
We need to make sure the client doesn't reuse an object ID until the
server has seen the destroy request. When a client destroys an ID
the server will now respond with the display.delete_id event, which lets
the client block reuse until it receives the event.
