Activity Diagram: Horizontal bars for split/join of parallel events

[b-fein]

Is your feature request related to a problem?

In activity diagrams it is only possible to place vertical bars for split/join of activities.

Describe the solution you'd like

I’d like to be able to rotate the element to another direction, especially to align the bar horizontally.

Describe alternatives you've considered

There is a second element I can drag-and-drop from the sidebar that represents a horizontal bar.

Additional context

[b-fein]

Implemented in #245