Added initial implemntation for default displayed field generator rule
Added merge helper functions
Merge remote-tracking branch 'remotes/cpinter-commontk/276-dicom-database-display-tables' into 276-dicom-database-display-tables

Conflicts:
	Libs/DICOM/Core/ctkDICOMDisplayedFieldGeneratorAbstractRule.h
Fixed rules (still WIP)
Merge remote-tracking branch 'remotes/cpinter-commontk/276-dicom-database-display-tables' into 276-dicom-database-display-tables

Conflicts:
	Libs/DICOM/Core/ctkDICOMDisplayedFieldGeneratorAbstractRule.h
	Libs/DICOM/Core/ctkDICOMDisplayedFieldGeneratorDefaultRule.h
	Libs/DICOM/Core/ctkDICOMDisplayedFieldGeneratorRadiotherapySeriesDescriptionRule.h
ctkDICOMDatabase::getCachedTags changed to return empty string if the cached field is empty or the field is not in the instance
Fixes in the insertion and update commands in ctkDICOMDatabasePrivate::applyDisplayFieldsChanges

Merge remote-tracking branch 'remotes/lassoan/276-dicom-database-display-tables' into 276-dicom-database-display-tables

Conflicts:
	Libs/DICOM/Core/ctkDICOMDisplayedFieldGeneratorAbstractRule.h
	Libs/DICOM/Core/ctkDICOMDisplayedFieldGeneratorDefaultRule.h
	Libs/DICOM/Core/ctkDICOMDisplayedFieldGeneratorRadiotherapySeriesDescriptionRule.h
Merge remote-tracking branch 'remotes/lassoan/276-dicom-database-display-tables' into 276-dicom-database-display-tables

Conflicts:
	Libs/DICOM/Core/ctkDICOMDisplayedFieldGeneratorAbstractRule.h
	Libs/DICOM/Core/ctkDICOMDisplayedFieldGeneratorDefaultRule.h
	Libs/DICOM/Core/ctkDICOMDisplayedFieldGeneratorRadiotherapySeriesDescriptionRule.h