Add support for resource conversion plugins in filesystem dock

[SaracenOne]

Based on a past discussion for a long overdue UX feature. Makes ResourceConversionPlugins avaliable to the filesystem dock, including allowing multiple files to be batch converted in one go and updating the usage of said resources in active scenes. Keeping in draft for a bit longer since I want to look at revising some of the API naming around detecting viable conversion plugins based purely on class name to avoid having to load the resource beforehand, and possibly look into introducing a warning dialogue explaining how conversion of types is a one-way operation.

That being said, I think the majority of the implementation is fine.

[SaracenOne]

Decided to revisit this and add the remaining things I wanted to add because, to be frank, the current workflow for converting standard materials to shader materials is kind of atrocious. It now gives you warning when you attempt to convert files from the filesystem dock, and although there are currently no instances where multiple conversion types are avaliable, it will now collapse the conversion choices into a submenu if more than one is present in the future.

I'm not happy about the fact that I had to add a new (and fairly reduntant) function to the API. I would have much rathered change 'handles' to a string type rather than a resource type, but I realize that doing so would break the API and subsequently break GDExtensions. As such, another function is added to ResourceConverterPlugins to check handling by type name rather than resource, meaning the resources won't have to be loaded when simply right-clicking on them

[AThousandShips]

Dedicated icon syntax

[SaracenOne]

Okay thanks, implemented changes recommend by @AThousandShips

[SaracenOne]

Okay, updated now

[KoBeWi]

I think handles() takes a whole object to allow filtering objects by more than only a type (although it's not really used in practice).

You can use this trick to achieve the same functionality with a low cost:

	Ref<Resource> temp = ClassDB::instantiate(p_type);
	for (int i = 0; i < resource_conversion_plugins.size(); i++) {
		if (resource_conversion_plugins[i].is_valid() && resource_conversion_plugins[i]->handles(temp)) {
			ret.push_back(resource_conversion_plugins[i]);
		}
	}

Instantiating an empty resource is much cheaper than loading it.

[SaracenOne]

Okay @KoBeWi and @AThousandShips, I think that should address most of the issues raised. One I'm not sure about is @KoBeWi's point about setting conversion dialog text in the constructor.

[KoBeWi]

The feature works correctly. I left some more style comments, but the code is good enough to be fixed in a follow-up.

[SaracenOne]

Updated again! @KoBeWi I wasn't sure if that was what you meant by an 'early return', but can you confirm that the change I made was what you were suggesting?

[KoBeWi]

Yes. You could also do return ret; to return empty Vector.

[akien-mga]

Thanks!