workspacesView: Use calculated workspace state for progress
It works just like before, except that this has the nice side effect
of not changing the workspace mode when leaving the overview from
the app grid state.
Rename ViewSelector to SearchController, since ViewSelector now effectively
only handles search. Rename the file correspondingly as well. I took the
liberty to sneak in some trivial style fixes.
Remove the dummy applications page that was introduced as a temporary step. Replace
the 'page-changed' and 'page-empty' signals with a 'searching' boolean property.
Remove ViewSelector.ViewsPage since it's now unused.
Now, we set WorkspacesDisplay's opacity directly, so there's no
need to have this parent → child glue code anymore. Remove it,
and bind the primary view's opacity with WorkspacesDisplay's.