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.
Now all classes and members are either exposed to bindings or marked
as "not available in Python bindings" in the docs.
Drop test thresholds to 0. Now it should be much easier to determine
what missing members have been added which are causing test
failures.