Copy files between different storage providers

[jesmrec]

This comes from release 1.0.0

In Files App, with two oC accounts attached:

1. From one account select a file and Move
2. As target folder, select the root of the other account
3. Do the same, selecting as target a non-root folder

Current:

2. File is copied, not moved.
3. Error raised:

Here we can check some @felix-schwarz comments:

https://github.com/owncloud/ios-app/issues/237#issuecomment-483163528

[michaelstingl]

Move to another storage is not usual. Copying is okay. We just shouldn't confuse the user.

[michaelstingl]

@felix-schwarz quick thing to fix?