conflict handling when filestate = BOTH_CHANGED

[bytinbit]

When BOTH files individually changed, we currently override the local file, but this can and should
later be handled as a user decision.

Implementation in settings with override / ignore / ask every time / rename

[bytinbit]

we agree: the simplest implementation is to just rename the local file and download the remote changed file anyway.

another issue has been opened to cover a "second stage": the user can determine in the config what should be done when both (remote, local) have changed.

[The-Compiler]

@bytinbit started some work on this in the filecache-both-changed branch. Some tests are still missing (we wanted some more fine-grained tests for the logic in syncing.py with the different file states), but otherwise it should be done.