[Alt-tab] A more general way to remember which window is selected.
[Alt-tab] Let windows-from-all-workspaces mode default to off, must be explicitly enabled.

You enable/disable it by pressing plus or minus, respectively.

The mode is now persisted through the current session, so it is remembered
between invocations, but not through a Cinnamon restart.
[Alt-tab] Make the secret 'thumbnails' Alt-Tab style work again.
[Alt-tab] Update quick help.