Addresses github issue swaywm#10

* Remove the concept of a workspace group, instead associate outputs directly with
  workspaces.

* Remove 'empty' state: clients will instead determine themselves if a workspace
  is empty once functionality is added either to this or the foreign toplevel
  manager protocol to determine which toplevels are on which workspaces.

* Add name event

* Replace number event with coordinates of arbitrary dimension

* Add deactivate request, since activating one workspace need not imply
  that others are deactivated

* Rename destroy event to remove.

* Improve descriptions

* Fix two minor bugs pointed out in reviews
* clarify meaning and optional-ness of coordinates.

To match the foreign-toplevel-management protocol.