KUrlNavigator: Fix Tab order
KUrlNavigator contains a lot of widgets which are dynamically shown or hidden under various conditions. This messes with the keyboard focus order which can lead to inane focus orders like losing focus to widgets outside of KUrlNavigator instead of going to the neighboring button.
This commit enforces a sensible Tab key focus order. Focus goes left-to-right through all visible child widgets. The leftmost such widget is set as the focusProxy() of this KUrlNavigator.
Best tested together with system/dolphin!851 (merged) for a mostly bug free tabbing experience.