[Build Addon] Force LTR direction on the treeWidget for RTL locales

Summary:
In RTL locales, the tree widget (showing target set and errors) is mirrored and
unusable, this should force it’s direction to be LTR.

Reviewers: #kate, #ktexteditor, sars

Reviewed By: sars

Tags: #kate

Differential Revision: https://phabricator.kde.org/D10003