Improve dashed bar line

Fixes several ugly gaps between bar line and span bar line. Previously the dashed bar line definition used vertically ligned up round filled boxes or the dashed line stencil relying on a certain condition.
Though, a dashed line stencil, vertically meeting a staff line, will always cause some sticking out or visible gaps, at least for default thicknesses of bar line and staff line. Thus we drop the possibility to use the dashed line stencil and do all with round filled boxes.

Add regtests.

This patch aims at improving dashed bar lines, also:
Closes #6666 (closed)