BreadcrumbControl: Fix wrong heading offset in RTL layout direction

Unlike anchors and Layout.*Margin properties, left/right paddings are not automatically mirrored. And even if they were, this case was just plainly inconsistent itself.

Fixes mis-positioned labels in Breadcrumb header style, such as in Plasma applet configuration window and in some System Settings KCMs.

Before / After (top/bottom):

Before	After