Add location hint for Parts' toolbars

Summary:
Fixes the order of toolbars and the orientation of the navigator
toolbar if other toolbars (except the main toolbar) are enabled.

BUG: 383916
FIXED-IN: 5.9.0

Test Plan:
Enabled various combinations of toolbars in various kontact components.
Navigator toolbar orientation as well as the toolbars' order is broken
iff the navigator toolbar and any toolbar other than the main toolbar
is enabled without the patch. Everything was fine with the patch.

Reviewers: KDE PIM, mlaurent

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D10644