protocol: Add DnD actions

These 2 requests have been added:

- wl_data_source.set_actions: Notifies the compositor of the available
  actions on the data source.
- wl_data_offer.set_actions: Notifies the compositor of the available
  actions on the destination side, plus the preferred action.

Out of the data from these requests, the compositor can determine the action
both parts agree on (and let the user play a role through eg. keyboard
modifiers). The chosen option will be notified to both parties
through the following two requests:

- wl_data_source.action
- wl_data_offer.action

In addition, the destination side can peek the source side actions through
wl_data_offer.source_actions.

Compared to the XDND protocol, there's two notable changes:

- XDND lets the source suggest an action, whereas wl_data_device lets
  the destination prefer a given action. The difference is subtle here,
  it comes off as convenience because it is the drag destination which
  receives the motion events (unlike in X) and can perform action updates.

  The drag destination seems also in a better position to update the
  preferred action based on things like the data being transferred, the
  place being dropped, and whether the drag is client-local.

- That same source-side preferred action is used in XDND to convey the
  modifier-induced action to the drag destination, which would then ack
  it, or reply with another action that's accepted (or none), this makes
  the XdndPosition/XdndStatus messaging very verbose, and synchronous
  because the drag source always needs to know the latest status/action
  for every position+action sent.

  Here it's the compositor which takes care of modifiers and matching
  available/accepted actions, this allows for the signaling to happen
  only whenever the actions/modifiers change for real.

Roughly based on previous work by Giulio Camuffo <giuliocamuffo@gmail.com>

Changes since v10:
- Narrow down the situations where wl_data_source/offer.accept requests
  are supposed to happen.

Changes since v9:
- Deferred the protocol errors to .finish after some IRC chat with Jonas,
  added further errors if actions API is used on selection sources/offers.

Changes since v8:
- Defined further the expected behavior on "ask", described the protocol
  errors that may happen. Fix more spaces vs tabs issues.

Changes since v7:
- Misc changes after updating the progress notification patch.

Changes since v6:
- Further explanations on wl_data_source/offer.set_actions, including a
  description of "ask" actions. Added protocol errors for unknown action
  values.

Changes since v5:
- Applied rewording suggestions from Jonas Ådahl. Dropped slot reservation
  scheme for actions. Fixed indentation and other minor formatting issues.

Changes since v4:
- Minor rewording.

Changes since v3:
- Splitted from DnD progress notification changes.
- Further rationales in commit log.

Changes since v2:
- Renamed notify_actions to set_actions on both sides, seems more consistent
  with the rest of the protocol.
- Spelled out better which events may be triggered on the compositor side
  by the requests, the circumstances in which events are emitted, and
  what are events useful for in clients.
- Defined a minimal common ground wrt compositor-side action picking and
  keybindings.
- Acknowledge the possibility of compositor/toolkit defined actions, even
  though none are used at the moment.
Changes since v1:
- Added wl_data_offer.source_actions to let know of the actions offered
  by a data source.
- Renamed wl_data_source.finished to "drag_finished" for clarity
- Improved wording as suggested by Bryce

Signed-off-by: Carlos Garnacho <carlosg@gnome.org>
Reviewed-by: Michael Catanzaro <mcatanzaro@igalia.com>
Reviewed-by: Mike Blumenkrantz <zmike@samsung.com>
Reviewed-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>