events: support calling external commands on root window clicks

Since xfwm is already listening to button events on the root window, there's currently no (practical) to catch these by other applications, e.g. presenting some root menu. A simple approach is just calling an user defined an external command.

(this doesn't make much difference when using xfdesktop, since it overlaps the root window by its own desktop window, thus mouse clicks don't even reach the root window).

Signed-off-by: Enrico Weigelt, metux IT consult info@metux.net