KUrlNavigatorButton: Use arrow as separators

Due to the feedback of regular separator not being good enough for separating folders, use an arrow separator instead.

This also refactors bunch of code around the button, so it should be easier to tweak over time.

Remove icons due to the added clutter. They would work with full height chevrons but those are not technically feasible at this time.

BUG: 501587 BUG: 501575 BUG: 501582 BUG: 501589 BUG: 501706 BUG: 501803 FIXED-IN: 6.14