Fix Title Flickering on Document Change

Summary:
Use new option from KF5 24 to disable KPart's title handling.
This prevents KPart to act on changed KTextEditor documents by
setting a title different to the one KDevelop wants to set.

Test Plan: Manual testing.

Reviewers: KDevelop, kfunk

Subscribers: kdevelop-devel

Differential Revision: https://phabricator.kde.org/D2125