* Fix bug reported by humdinger: when the volume which is
currently shown in the file panel is unmounted, the file
panel's window was closed and even deleted. This is not only
inconvenient but also a use-after-free bug waiting to happen:
an application which keeps the BFilePanel around to show it
again later (as recommended in the BeBook) cannot know that
it has become invalid while it was hidden (the destruction of
the window happens even when it's not shown).
* When receiving an unmount event for the currently shown volume,
we now reset the view to the home directory.
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