First version of workaround getting image of minimited windows.

The workaround is as follows:
* Set up signal handlers to get notified about changes of window
* Unminimize window
* If window is visible it will be activated by design (we shoud reactivate last active window - but this is _NOT_ implemented now!)
* Minimize window again
* Stop watching for changes of window by disconnecting signal handlers

Please notice: The daemon mode of xfdashboard is _NOT_ tested now!