Improve split wall (single wall) vertically.

The following things have been added to the command "Split Wall Vertical":

• A jig showing measures.
• The possibility to regret selected breakpoints with "Undo"
• The possibility to show mass.

Note, the mass calculation isn't exact, the following is assumed or isn't considered:

If the original element has lift fillings on the left/right edge, the new elements also assume to include these lift fillings.

If the original element has lift fillings on the top edge, the new elements also assume to include these lift fillings, but a maximum of 2.

Any "thickenings" in the selected endcap aren't considered.