[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.