[Alt-Tab] Enable thumbnails-behind-icons-if-more-than-app-of-the-same-kind-is-open mode for 'thumbnails+icons' style
[Alt-tab] Move cleanup duty from refresh to show.
[Alt-tab] Add new keyboard commands to set/unset windows-from-all-workspaces mode

+ (plus): set all-ws mode
- (minus): unset all-ws mode

Special care has been taken to retain the current selection when the list of icons
is refreshed.

Currently not persisted, only in effect until Alt-Tab is closed.
[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.
[Alt-tab] Fix a regression to the zoom function.
[Alt-tab] Shade ignored window icons.