[layout.js] Defer the actions taken on monitors-changed event.

Cinnamon will almost invariably crash when I add a secondary
monitor to my laptop, with no way to recover. Hopefully, a small
timeout may help.
[Alt-Tab] Size the icons according to the number of windows on the current workspace (take some account of the number of windows on other workspaces).
[Alt-Tab] Show a window thumbnail behind the icon.