Refactor scripting UI logic

[WiIIiam278]

Scripting is fairly buggy at the moment and is in need of a refactor; specifically, decoupling the UI drag/drop/copy/paste logic from the actual validation and processing logic.

[jonko0493]

Putting this in the v0.3 milestone for now but I'd have no problem kicking it to v0.4 since it's a pretty big task.

Related existing bugs:

• Script editor preview doesn't update on parameter changes #282
• Copying DIALOGUE command doesn't bring speaker property along with it #284
• Copy/pasted commands bring their previews with them #285

[jonko0493]

I think we should keep this as part of v0.3 as we can work on it as part of #371.

[jonko0493]

This is in-progress and looking extremely good so far. Avalonia is making the script editor much more straightforward and so I think we'll be able to handle these changes more easily.