Add custom methods for adding editor translations

[KoBeWi]

Closes godotengine/godot-proposals#6885
Closes godotengine/godot-proposals#1262

Test addon with translation:
translationtest.zip

The implementation is as described in the proposal, but it has one problem - it doesn't support translation context. Coupled with the fact that duplicate messages trigger a warning, there is a chance for a conflicting messages in translations of multiple plugins.

Alternative suggested approach is to modify TranslationServer methods to work in editor. It would still likely require a separate method, because AFAIK TranslationServer handles only project's translations (I think even in editor?). Editor translations have some dedicated methods that work differently.

[YuriSizov]

I'll repeat my comments from RC.

I don't think this should be done using new custom methods. TranslationServer is already available to scripting, and it also supports loading multiple translation resources and merging them. Adding more methods to EditorPlugin and reimplementing some of that logic doesn't make sense to me. I'd prefer we ensured that TranslationServer just works in the editor. (Last time I tried, it was operational, but completely clueless about which locale the editor used, and translating messages didn't work.)

[timothyqiu]

I think this is generally a better approach than #82158.

Instead of writing translation logic in EditorPlugin, I think we should put that in TranslationServer. That is, make tool_translation manage multiple translations, similar to translations:

godot/core/string/translation.h

Lines 79 to 82 in c12d635

	HashSet<Ref<Translation>> translations;
	Ref<Translation> tool_translation;
	Ref<Translation> doc_translation;
	Ref<Translation> property_translation;

Then EditorPlugin code would just be a thin layer of redirection.

[KoBeWi]

Updated with the above suggestion. Sorry for the delay 🙃

[KoBeWi]

Found a problem. When you add translation, it will always be used for translating matching messages, even if the locale doesn't match. How it should be handled? Should non-matching locales get ignored? (i.e. not added) Or it should be simply documented?

[akien-mga]

Superseded by #95787.