Fix visual editor event copy/paste issues

[salianifo]

Closes: #2455
Closes: #2407

When pasting into an empty timeline, the paste position would be set to -1, making the first item pop down to the end of the list.

[salianifo]

Since I'm looking at the copy/paste features of the visual timeline, I noticed there's a few other things that need to be fixed as well, such as nested events not being nested when pasted.

[salianifo]

I noticed that when you paste into a timeline with events (but none selected), it pastes right before the last event instead of after it. This last change fixes this as well. I think the other copy/paste/move issues need discussion, and should be addressed in another PR.

[Jowan-Spooner]

Why did you change it from using from_text to _load_from_string? It sounds correct to me, but it makes me wonder if we should also use _store_as_string() instead of to_text(), to avoid the translation ids getting lost. Was there something specific that mad you change it?

[salianifo]

Yes, when you have custom defaults set, they get loaded in _load_from_string which then calls from_text. Without this, if you copy an event that has and uses custom defaults, when pasting it will paste the original defaults instead. My tester set a default audio bus for Voice events, but when copying and pasting a brand new event it would paste it with Master instead of Voice for the audio bus.

Edit:
As for the second point, that is probably a good idea, as well. I haven't used translations at all.

[Jowan-Spooner]

Looks good to me. Tysm!