window: Avoid focusing during workspace changes
We can land inside meta_window_focus() in the middle of changing the
window workspace, because some signal handler of MetaWorkspace's
"window-removed" signal triggers a focus. This can cause a crash in
g_assert (link) when updating the MRU list because we still think
we're on the old workspace when actually we are already removed from
this workspaces MRU list.
To avoid crashes like this, bail out of meta_window_focus() when we're in the middle of a workspace change.