This change consolidates more unit handling and conversion into
QgsUnitTypes.
Additionally, UnknownUnit was renamed to UnknownDistanceUnit.
All methods which accepted QGis::UnitType parameters have been
updated to take QgsUnitTypes::DistanceUnit instead.
ALso remove the unit handling methods toLiteral, fromLiteral, tr,
fromTr, and fromUnitToUnitFactor from QGis. Their corresponding
counterparts in QgsUnitTypes should be used instead.