settings-dialog: Clear shortcut in treeview

Bug: 12802

Clearing a shortcut in the UI would rightfully remove the shortcut
definition, but would not clear the corresponding entry in the treeview.

Clear the shortcut entry in the treeview so that users are not left
thinking the clear action didn't do anything.

Signed-off-by: Olivier Fourdan <fourdan@xfce.org>