[enhancement] provide (emergency) possibility to move invisible window

[Karl-Philipp Richter]

If in case of malious behavior, windows might get moved out of visible area of the desktop. If this happens (more often in VMs than on PCs), the user has no possibility to move this window back into the visible area. One might argue that the reason for this is malious behavior and bugs should be fixed instead of introducing an ambiguous function. I guess, it wouldn't hurt though to add e.g. a context menu to the window overview (where all windows of one application are displayed in a raster) to provide a function to restore the position to the center to the screen or the intial position.

I'm using 7.1.2 on Ubuntu 13.10