status/keyboard: Reset menuItems and Label objects on change

In the current code it could happen that we've menuItems and indicatorLabels
for sources that aren't anymore around, because in case a source is removed
we don't cleanup the their container objects.
Also, we should nullify InputManager's _currentSource when sources change
or it might point to some invalid data again.

So it could happen that we try to access an invalid menuitem or label
if a source change happens mentioning a source that has been deleted.

https://bugzilla.gnome.org/show_bug.cgi?id=788931