[systemsettings] Remove AboutSettings::model()

Has been returning strange information. Better can be now asked from
SSU DBus interface with displayName method.
[systemsettings] Remove const on string return values

Doesn't provide anything but noise.
[systemsettings] Remove manufacturer and productName from aboutsettings

QtSystems returning invalid data on these. Better can be now asked from
SSU DBus interface with displayName method.
[systemsettings] Add support for fetching software version in aboutsettings