Custom Theme per Character

[Jowan-Spooner]

You can now select a theme for each character.
If the character says something the theme will be loaded. If then a character without a custom theme will says anything, it will go back to the current theme.

An implementation of #231.

[zaknafean]

Bug Report:

1. If you delete a theme that's in use by a character, it breaks weirdly. The characters theme field will correctly say 'No custom theme' but your game will crash when it gets to that character until you choose a new theme. That fixes it, but even restarting Godot doesn't otherwise fix it.
2. There doesn't appear to be a way to remove themes once you choose a custom theme

So far thats it! I'll see if I can't dig more bugs out of it.

[Jowan-Spooner]

@coppolaemilio if you don't test this soon, I'll just merge it!