This commit makes it possible for a single client to have multiple data devices
for the same seat. This fixes issues with Firefox.
This mainly removes wlr_data_source.offer. We make sure we create one data
offer per device. We now make the offer inert when the source is destroyed.
Fixes the second half of https://github.com/swaywm/wlroots/issues/1041
Since the source doesn't always come from a client, this field
doesn't make sense. It is replaced by a new "finalized" field in
wlr_client_data_source. This is used to make sure set_actions is
not sent after start_drag has been sent.
A check in data_offer_choose_action has been removed: if an offer
has been sent then start_drag has been called, no need to check.
I also wanted to add a check for wl_data_source.offer, but it
turns out (1) this isn't in the spec (2) it breaks GTK+.
This is some preliminary work for Firefox on Wayland compatibility.
