panel: add `PanelMenu.Button` menu when `menu-set` is emitted
When a PanelMenu.Button
is added to the statusArea
, its menu is
added to the PopupMenuManager
if it has one at the time. If a menu is
added later or a new one set, the new menu is never added.
Move the call to PopupMenuManager.addMenu()
from _addToPanelBox()
to _onMenuSet()
, which is called when the PanelMenu.Button
is added
and whenever it emits the menu-set
signal.