Thumbnail windowclone not properly disconnecting signals from app realwindow.

Details:
We can't use the windowclone's source on the parent window because we've reverted to using the realWindow texture to fix a previous bug where the parent window disappeared when a modal dialog was opened (see commit 82a2099da4eb1fcb7485b91b4a18e8fa97bf561c).
Fail to remove taskbar icon when minimized window is closed.
Update readme, metadata, and installation zip for Gnome 3.12 v22.