[Alt-Tab] The thumbnails behind the icons are really only needed when there is a need to visually disambiguate between more than one window having the same icon.
[notification applet] Handle destroyed notifications gracefully.

This should fix the bug which stops the applet responding to mouse
clicks.
[connector] Make safe for repeated disconnections.
[Alt-Tab] Make the switcher "hide" with low opacity, toggled with the 'h' key.
[Window Manager] Sense when a window is marked urgent or demanding attention.

Post a notification and timer-activate windows demanding attention.