Fix support for dead keys in foldertreewidget filtering

Summary:
CCBUG: 415850
FIXED-IN: 5.13.2

Test Plan:
I can jump to a folder called abédêf with FR layout.
Doesn't seem to work with compose key though.

Reviewers: mlaurent

Reviewed By: mlaurent

Differential Revision: https://phabricator.kde.org/D26682