On concept page, ordering of narrower concepts is not correct wrt accented characters

[tfrancart]

At which URL did you encounter the problem?

http://erato.sparna.fr/scolomfr/scolomfr-voc-000-3-2/fr/page/?uri=http%3A%2F%2Fdata.education.fr%2Fvoc%2Fscolomfr%2Fconcept%2Fscolomfr-voc-015-num-099

What steps will reproduce the problem?

1. Go to above URL
2. In the narrowers section, narrowers starting with "é" are at the end of the list

What is the expected output? What do you see instead?

narrowers starting with "é" should be placed after narrower starting with "e".

In ConceptProperty::sortValues(), strcoll should be used instead of strnatcasecmp in combination with lowercasing to have case-insensitive + locale-based ordering.

PR to follow.

[osma]

This caused a regression in PTVL. Previously the classes (concepts) in the classification were ordered numerically by notation code, e.g. P1 comes first and P10 comes after P9:
http://finto.fi/ptvl/fi/page/?uri=http://urn.fi/URN:NBN:fi:au:ptvl:

But after this fix, P10 comes first and P1 is after P19:
http://dev.finto.fi/ptvl/fi/page/?uri=http://urn.fi/URN:NBN:fi:au:ptvl:

I'm not sure if it's possible to do a correct sort in both cases (accented characters + natural sort for numbers).

[osma]

I made a separate issue for the PTVL problem, so this one can be closed.