Since commit ef1e27966db2 turned DashItemContainer into an StWidget,
the destroy() method overrides the ClutterActor method, which is at
the very least bad style. Instead, follow the usual pattern of using
a ::destroy handler.
dash: Make sure item labels are only destroyed once
Labels are currently destroyed from both animateOutAndDestroy()
and destroy(), which now (rightfully) triggers a gjs warning. As
the label is created unconditionally since commit 36e5ae4a250,
mirror that and always release it in destroy() and hide it
elsewhere.
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.
These end up emitting item-drag-end/window-drag-end pretty much
without checks. Given the MetaDnd object may end up emitting
::drag-leave as a result of the plugin ending its grab, this
would result on spurious emission of those events and subsequent
warnings.
For extra paranoia, the _inDrag variable has been split into
_inItemDrag/_inWindowDrag so we can't cross the streams.
