schemas: Add switch to last/first workspace keybindings

It makes sense to have <super>+Home to switch to the first workspace by
default, because we already have <super>+pgup and <super>+pgdn to switch
worsplaces relatively.

It also makes sense to have <super>+End to switch to the last workspace, which
varies when using dynamic workspaces.

https://bugzilla.gnome.org/show_bug.cgi?id=659288