Fix bug related to attached modal dialogs in thumbnail windowclone.

Details:
The bug was especially noticeable in virtualbox where you have modal dialogs open from 
other modal dialogs and not just the parent window. This caused Gnome Shell to crash.
Fix issue where removing 'removeBarrierTimeoutId' from the mainloop throws window 
manager warning messages.


Details:
The _removeBarrierTimeoutId was not being reset to 0 and thus throwing ..
Window manager warning: Log level 8: Source ID ### was not found when attempting 
to remove it.
Need to test that rect variable is valid before using it to set actor position.
Fix issue where removing 'windowChangedTimeout' from the mainloop throws window 
manager warning messages.

Details:
windowChangedTimeout was not being reset to 0 and thus throwing ..
Window manager warning: Log level 8: Source ID ### was not found when attempting 
to remove it.
Fix thumbnail windowclone bug causing parent window to disappear when modal dialog is 
opened.

Details:
The clone should use realWindow.get_texture() as source. This should probably be 
reported upstream as a Gnome Shell bug.
Update readme, metadata, and installation zip for Gnome 3.12 v21.