mtp: Refactor source/dest file/dir validation logic

There is a fairly elaborate set of conditions that need to be
respected when copying or moving files. Right now, we've got this
logic duplicated in four different places (push/pull/move/copy),
so it's worth the effort to consolidate that logic into a single
place.

Along the way, I'm deliberately ignoring the ability of CopyObject
to copy folders. By forcing the client to do file-by-file copies,
we can at least notify when each file completes, to provide slightly
better progress reports. There is a performance impact - I suspect
that a large directory of small files will end up being orders of
magnitude slower, but for a small number of files, it's negligible.

I have not done this for MoveObject, because that is fast unless
the source and target are on different Storages - and devices
with multiple storages are getting less and less common.

https://bugzilla.gnome.org/show_bug.cgi?id=794388

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