More resilient file saving by a two-stage file save process

[flxzt]

As mentioned in #1096 it would be very nice to improve the save logic by first saving to a temporary second file and overwriting the first when writing the file actually succeeded.

Right now we are directly overwriting the file, which can corrupt files when the app exits unexpectedly while saving.

[RayJW]

While the discussion is already at it, how exactly are the files overwritten? From a security standpoint, overwriting can be a risk by making deleted data recoverable (see aCropalypse). Not sure how relevant that is, but I don't know the use case for Rnote of all people.