Activity diagrams: Add horizontal fork node

[minrows]

Checklist

• I added multiple screenshots/screencasts of my UI changes

Motivation and Context

This PR introduces a Horizontal fork node for Activity Diagrams.
Issue Number: #240

Description

Horizontal fork node for drawing Activity/Sequence diagram was not possible in Apollon.
In order to solve this problem, a new Horizontal fork node is now introduced.

Steps for Testing

1. Clone a repo
2. Execute commands: yarn install yarn build followed by yarn start
3. Goto the application, navigating to URL: localhost:8888
4. Select Activity Diagram from Diagram Type
5. Observe new horizontal activity node ready to be used

OR

1. Go to test server
2. Observe new element Horizontal fork node in the list of Activity/Class Diagram

Screenshots

Additional Info:

Changes included in Release version: 2.10.4-alpha.2

[akesfeden]

Tested locally, horizontal activity node is visible and usable. When I try to resize the node, the cursor shows diagonal arrows but I think having an horizontal arrow would be more correct. But maybe this is outside of the scope of this PR because it also happens with vertical activity node.

[minrows]

Yes @akesfeden , this PR is just for the addition of a new element, which is a Horizontal Fork node, and that behavior is out of scope.
But thanks for the feedback. That can be enhanced on separate PR. 👍

[canberkanar]

Tested on Apollon TS1 in the testing session. Although there are no junction points on the corners, the algorithm still connects the arrows there on some occasions as well. Developer informed that it is intentional so the PR is approved.

[ole-ve]

Tested during TS, worked fine 👍

[xlieA]

Tested during testing session. Works as described.