Add support for BPMN groups and adapt subprocess visualization

[matthiaslehnertum]

This PR intends to add support for groups within BPMN diagrams and adapts how subprocesses are rendered to comply with the BPMN 2.0 standard.

Checklist

• [x] I documented the TypeScript code using JSDoc style.
• [x] I added multiple screenshots/screencasts of my UI changes
• [x] I translated all the newly inserted strings into German and English

Motivation and Context

Groups allow for easier entity management in larger BPMN diagrams - therefore this change adds support for them. Additionally, to comply with the specification of the BPMN 2.0 standard, the way how subprocesses are rendered is updated.

Steps for Testing

1. Add a group (box with dashed outline) to the canvas
2. Drop other elements inside the group
3. Trying dragging the group around to assure everything inside the group is moved as well
4. Delete group to ensure contained elements are deleted as well
5. Drag subprocess onto the canvas and make sure it is displayed as expected

Test Coverage

File	% Stmts	% Branch	% Funcs	% Lines
main/packages/bpmn/bpmn-group	94.73	100	66.66	94.73
bpmn-group-component.tsx	100	100	100	100
bpmn-group.ts	85.71	100	50	85.71
main/packages/bpmn/bpmn-subprocess	95.45	100	66.66	95.45
bpmn-subprocess-component.tsx	100	100	100	100
bpmn-subprocess.ts	80	100	50	80

Screenshots