Implicit fork & join

[sfmskywalker]

This PR enhances the Flowchart activity by supporting implicit forks & joins.

By default, implicit joins always employ a "wait all" strategy.
If you need a "wait any" join mode, use the explicit "Flow Join" activity and configure its mode with "Wait Any".

The following screenshots displays what's now possible:

It showcases two implicit forks, one implicit join and one explicit join.
When running this workflow, the output will look like this:

Start
Line 2
Line 2a
Line 1

<after 5 seconds...>

Will happen
End