Project plugin: sort the tree view case insensitively

Summary:
This matches how the Filetree plugin sorts its tree view, so that
using the Next/Previous Document (from Filetree plugin) actions while
looking at the Projects panel doesn't skip/jump over files.

BUG: 403997
FIXED-IN: 19.08.0

Test Plan:

• Create a git repo that has three files with mixed case names, e.g.:

a
B
c

• Open kate and look at the filetree panel and the projects panel, how

the files are sorted should match.

Reviewers: Kate, cullmann, dhaumann

Reviewed By: Kate, cullmann

Subscribers: kwrite-devel

Tags: Kate

Differential Revision: https://phabricator.kde.org/D20319