Remove IfdId from the published interface
Together with #719, this will allow more changes to the makernotes without affecting API/ABI compatibility in the future. Since makernotes are rather dynamic, it's important to be able to make such changes.
Notably, classes ExifTags, ExifDatum and ExifKey will be affected by this change and applications which use the related method will need to change.
As a replacement, a method using the group name instead of the IfdId is planned where needed.
#721: Removed IfdId from the published API. ExifKey now gives access to reference tag data. (Needs some polishing.)
#721: Renamed ifdItem to groupName and big internal clean-up: consolidated groups and IfdIds.