restoring a file from trashbin overwrites existing file without notice

[individual-it]

on webUI of current ocis master

1. upload a file called file.txt
2. delete file.txt
3. upload file.txt again but with different content
4. restore file.txt from trashbin
5. check the content of file.txt

file.txt is overwritten with the content of the trasbin version, the version in the users space is lost

Expected behaviour:

- the error message should give the reason "cannot restore in the original location XXX, a file with the same name is already there."
- and/or (even better) suggest to rename things to resolve that conflict, then restore.

this supersedes the behaviour in #139