Add a toggle-overview keybinding

This is a new, regular keybinding for going to the overview.
The default binding is Super-S, which goes well with Super-A
for going to the application grid.
This is separate from the existing panel-main-menu keybinding,
so that we can keep Alt-F1 opening the main menu in classic mode.

https://bugzilla.gnome.org/show_bug.cgi?id=698251