Add custom events, destroy action, and expand 2D demo

[Lexian-droid]

• Added custom event creation:
  • Action: Call Custom Event
  • Event: On Custom Event Called
• Added Destroy node action
• Added a variety of new features to the demo 2D game scene

[CG-Tespy]

While testing the new platformer demo, I found this:

That aside, I really like the idea of custom events! I believe it'd be better if rather than having the user type in a name for calling or responding to such an event, they instead drag and drop a Resource on disk to the action input ui or something. This would prevent bugs caused by typos, and also reduce the amount of names the user would need to keep track of in their head. If you'd like, I can offer some implementation ideas.

Edit: nevermind, probably best to fix the issues with typing in the names in a separate PR