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.