cpu: add voltage control

Easy voltage control is easy. I am not using the preference handler
here since its not enough flexible to solve this particular task.

Also notice the set on boot functionality is not implemented with this,
as usual.

Signed-off-by: Blechd0se <alex.christ@hotmail.de>
helpers: make the preference handler a bit smarter

The current PreferenceHandler worked very well for a dictionary
wich contained some files(parameters).
It couldn't handled single paths/files for some settings very well.

This problem should now be fixed, since it can either use
genPrefFromFiles() or genPrefFromDictionary() to build preferences
either from files or dictionaries. It makes it much easier to work
with and also a bit more flexible.

Signed-off-by: Blechd0se <alex.christ@hotmail.de>
cpu: fix a NPE during min/max frequency change

If the object from the preference returns something
like "Unavailable" we would end up in a NullPointerException.

Signed-off-by: Blechd0se <alex.christ@hotmail.de>
helpers: allow to change the font for each prefence

I am not sure yet how i will use this properly, but giving
an option to change the font on the fly is really nice.

Signed-off-by: Blechd0se <alex.christ@hotmail.de>