always drop priviledges when opening a directory

[ygerlach]

Fixes: #517

Always drop privileges when opening a file manager.

• For viewing timeshift logs user privs should be enough.
• For viewing snapshots user prives should mostly also be enough (unless you want to view /etc/shadow or other users files, ...)
• users can elevate their privileges on their own if they really need it

EDIT:
Fixes: #520