Enable numericMode for sorting document list in filetree

Turns on "natural" sorting for the document list in the filetree addon,
so that e.g. 10 comes after 9, not before.
That's how the KDE4 version behaved too.

BUG: 375676
FIXED-IN: 17.04.1
Differential Revision: https://phabricator.kde.org/D5448