[systemsettings] Add dedicated charger USB mode. Contributes: JB#15195

Have the dedicated charger mode forwarded from qmsystem so we can
use it in the settings.

Signed-off-by: Philippe De Swert <philippe.deswert@jollamobile.com>
Merge pull request #28 from philippedeswert/master

[systemsettings] Add dedicated charger USB mode. Contributes: JB#15195