Show all windows mode freezes when external monitor is disconnected

[cmonagle]

Note: this isn't a clean install but I haven't done anything too wacky, if someone could confirm this isn't something I've done that would be great :D.

Steps to reproduce:

1. With external monitor attached, show all windows (ctrl+down)
2. Wait for monitor to go black
3. Disconnect external monitor
4. Gala is trapped with windows in show all mode split on the one screen

Temporary fix described here: https://elementaryos.stackexchange.com/questions/747/altdrag-stopped-working-how-can-i-restart-a-window-manager