Previously we would compare the last focus's workspace with the new
focus's workspace to determine if we need to emit an IPC
workspace::focus event. This doesn't work when moving the focused
container to a new workspace.
This adds a workspace property to the seat which stores the last emitted
workspace::focus workspace. Using this method, after moving the
container, refocusing it will trigger exactly one workspace::focus
event: from the old workspace to the new workspace.
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