Fix name grouping feature for cyrillic names

Summary:
All files and folders with cyrillic names are placed in latin 'A' group. This patch fixes this issue.

CCBUG: 406867

Test Plan:
Make grouping by name in the dir with cyrillic files or dirs in it.
See screenshots below.
Before {F6985465}
After {F6985459}

Reviewers: #dolphin, ngraham, cfeck, elvisangelaccio

Reviewed By: #dolphin, ngraham, elvisangelaccio

Subscribers: hein, cfeck, ngraham, elvisangelaccio, kfm-devel

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D22303