Fix for Issue #193 (Desktop Icons Remain Highlighted when Mouse Cursor Leaves Them via Taskbar)

Uses a mechanism from GNOME/gnome-shell@d903e831 Should be reviwed for impact on overall performance of the desktop environment. Detected a similar issue independent of Issue #193 already present before this change. When the mouse pointer is moved back from a dock icon to a desktop icon, the hover effect disappers after a short delay. Most likely an issue in the dock. Will create a new Issue for this.