overview: Protect ::drag-end handlers

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.

https://bugzilla.gnome.org/show_bug.cgi?id=784545