Remove the dummy applications page that was introduced as a temporary
step. Replace the 'page-changed' and 'page-empty' signals with a 'search-active'
boolean property.
Remove ViewSelector.ViewsPage since it's now unused, and all the page handling
mechanism. At last, since we don't use any ShellStack features anymore, simply
make it a St.Widget with a ClutterBinLayout as the layout manager.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1667>
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