Fix regression with moving floating windows

Fix regression with moving floating windows

This fixes a regression introduced in 6d983b5. Consider 2 outputs:
fake-0: workspace '1'
fake-1: workspaces '2','3'
Workspace 1 focused, workspace 2 visible.
Open a floating window in 1 and move it to 3. Now, the floating window
appears in workspace 2 and disappears once focus is switched to
that workspace.

Instead of focusing 'old_focus' which might refer to a container in a
different output, we should restore focus by focusing the previously
focused workspace of the output.