Relocate IDataViewColumn::Alignment

* Relocate IDataViewColumn::Alignment enum to DataViewColumn
class.

* Change alignment type from enum to uint32 and use existing
Anchor values for the class alignment enum.
VisualUnitTest: Default to not overwrite reference image sets

* Introduce --force command line option to override the default.
Misc clean up of VisualUnitTest feature
Rename VisualTestSet to ImageTestSet

* Rename VisualTestSetTest to ImageTestSetTest