Allow arbitrary folders to be synchronized

Currently the nextCloud app will create a one to one folder representation of the remote server when synchronizing a directory. However it would be really helpful on a mobile device to map a device directory to a remote server directory.

A use case would be syncronizing a workspace directory of an app on the mobile device with a dedicated directory path on the remote server. For instance a directory with pictures often edited by a mobile app synchronized with a subpath of 'Photos' on the remote server. Or you want to synchronize a shared folder with music into your music library directory on your mobile device...

This would assume issue #19 to be working, such that changes on the mobile device are propagated to the remote server.

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