Move the HotCorner class from panel to layout, and make the panel
manage its own HotCorner.
Stick the panel's HotCorner into the Activities button actor (rather
than separately floating above it), so that hover tracking on the
button works properly without needing hacks in HotCorner.
https://bugzilla.gnome.org/show_bug.cgi?id=645759
Remove ShellGlobal's monitor-related methods, and have
Main.layoutManager provide that information instead. Move
Main._relayout() to LayoutManager, and have other objects connect to
the layout manager's 'monitors-changed' signal to know when the screen
geometry has changed.
https://bugzilla.gnome.org/show_bug.cgi?id=636963
