Shorten file paths to reduce filename too long errors

[schnatterer]

Especially when backing up to a nested folder there is a certain risk of hitting the maximal path length of the file system.

We could reduce the risk when changing the backup folder strucutre from e.g.

org.kde.kdeconnect_tp/data/data/org.kde.kdeconnect_tp

to

org.kde.kdeconnect_tp/data/data/

Same goes for sdcard.

This makes restoring more complex but could well be worth it.

In order to not break compatibility with existing backups, we should check the existence of another app-named folder first. This applies to both, restoring and backing up.