Use __root__ as id placeholder for filesystem root
/
is used as path separator, so root ends up being empty leading to
weird stuff like partitions//usedlevel . It also cannot be translated so
we end up with an empty category for root. Since "root" can be a valid
mount point, the placeholder adds some extra characters.
In addition, this changes the "All" sensors to not be displayed as "/all".
Edited by Arjen Hiemstra