Repaint area of deleted shadowed windows (fixes #27)

Hi, this is a fix for the bug where the network manager applet tooltip's shadow didn't disappear after the tooltip disappear.

The bug also happened for pretty much any tooltip from any GTK3 application, e.g. brasero

The fix code is checking if a window with a shadow has just been deleted, and if so, it is repainting the area of the screen that was occupied by the window.

Best regards :)