Align lower line of bar decoration to border width

Align lower line of bar decoration to border width

In normal border style, two horizontal lines are drawn to outline the
bar decoration. The lower line leaves a gap to the left and right to
align with the border. This gap was hardcoded to 2 pixels. Now it
matches the current border width, if any.

In stacked layout, only the bottommost bar is aligned. In tabbed layout,
no gaps are left in order to close the outline and make the notion of a
tab more clear.