Fix drop on mounted after start

[jeremypw]

Fixes #2465

Sets target file to null when bookmark not mounted and not accepts drop of target_file if null.

NOTE: Ignore first commit - this solution is not reliable as the File property is_mounted returns false for local files at the moment. Better to rely on presence of mount object in bookmark.

[danirabbit]

I guess this technically fixes the issue but I don't think this is the expected behavior. It seems like the expectation is that the drive would be mounted so that the drop operation succeeds.

If that's significantly more difficult, I can see this as like a hotfix, but I don't think it really solves the problem as far as giving the intended result

[jeremypw]

@danirabbit Fair enough - I'll work on automounting. I note that there is one issue that this does completely fix? Are you willing to merge on that basis and I'll submit a further PR in due course?

[danirabbit]

Yeah, I'm okay with merging this for now! Sorry I thought I left an "approve" comment. Should I open an issue report about mounting for a drop operation?

[jeremypw]

I've left a comment on #2390 making it clear that the mount should be automounted so I do not think another report is required.