Resolve #40 language switching

[RailKill]

Language switcher should work for the following:

• SentenceNode
  • Sentence
  • Voiceline
• ChoiceNode
  • Option

The class Localizable which extends Property is added. Basically, values in Localizable are always stored as Dictionary, where it will automatically get the currently selected language from the global language switcher. For example, if you have English and Spanish in the language switcher, and have Spanish selected, setting hola as a value in Localizable will be stored internally as:

{"Spanish": "hola"}

This dictionary is outputted only on _to_dict(), but if you're just calling localizable.value, it has a getter to automatically retrieve the correct value based on the currently selected language. When selecting a language, it will trigger a GlobalSignal to refresh all node previews and side panel so that it will use the corresponding language's values.

The .json and graph_edits now store an additional languages variable. Each graph_edit keeps track of the current_language_index to support tab-switching. Undo/redo should automatically switch languages when needed.

Example JSON Structure: SentenceNode

Before

"Sentence": "Hello this is a test",
"Voiceline": "eng.mp3"

After

"Sentence": {
    "English": "Hello this is a test",
    "Spanish": "hola esto es una prueba"
},
"Voiceline": {
    "English": "eng.mp3"
}

Top-level addition:

"Languages": [
    "English",
    "Spanish"
]

Notes

For now, the following features are missing but could be useful in future:

• No hotkey to switch language quickly.
• No quick way to compare multiple languages side-by-side.

[atomic-junky]

I love this feature, thank you so much for implementing it!
The only little problem I see is when you rename a language, you have to reselect it for the name to appear in the toolbox. And the selected language should have a different stylebox.

I also got an error on line 56 of monologue_dropdown.gd saying that item_id is a String and it cannot be converted to an int. It's always appening when I select a node.

[atomic-junky]

In fact, I think it's because of the redesign of the characters. I tested with a new file and everything works. My bad.

[atomic-junky]

Looks good to me👍🏼