Make label-based kind recognition case-insensitive

[jdutant]

At present localized labels are recognized:

Théorème
: La somme des angles d'un triangle est égale à deux droits.

But in a case-sensitive manner. In English kind recognition appears capitalization-insensitive because both Theorem (the label) and thereom (the internal kind name) are recognized. This will be confusing to users; better make label recognition case-insensitive.