Fix input gesture identifier normalization. (#6962)
Previously, arbitrarily ordered items (e.g. multiple keys) were reordered using Python set order. However, different input ordering could incorrectly cause different output in some cases due to hash bucket conflicts. For example, dot4+space and space+dot4 incorrectly produced different output.
In addition, these items were reordered *before* conversion to lower case instead of after, which could also incorrectly result in different output.
Now, the entire identifier is converted to lower case and arbitrarily ordered items are then sorted by character.
The InputGesture.identifiers property has been changed so that it is no longer normalized. Instead, there is now an InputGesture.normalizedIdentifiers property which returns normalized identifiers.
This was done to make normalization changes easier to apply and to eliminate the potential for normalization errors in InputGesture subclasses.
The InputGesture.logIdentifier property is now deprecated. InputGesture.identifiers[0] should be used instead.
Finally, various InputGesture implementations have been changed to remove normalization code. In some cases, this may result in nicer reporting in input help mode.
