Allow images to be imported "for editor use" and respect editor settings

[YuriSizov]

Closes godotengine/godot-proposals#5034.

This is implemented pretty much as I've described in the proposal. There are now two new import options that let the SVG icons to be imported for editor use and either scale, or convert colors, or both. We still need to document somehow which colors do we use, and there are some other limitations to the color conversion logic as it is currently implemented. But scaling is straightforward.

One limitation which I don't know how to fix without dirty hacks in core is that if you change the polarity of the editor theme you need to restart the editor or manually reimport the image for it to update. Changing scale already requires a restart, so it's not a big deal. There is already a hacky thing in core resource importer, but it shouldn't be too bad because we sometimes allow TOOLS_ENABLED in core and I think in this case this is a valid need.

Anyway, DEMO TIME!

godot.windows.tools.x86_64_2022-08-26_20-03-42.mp4

This work is kindly sponsored by the Dialogic project.

[YuriSizov]

From RC:

reduz 12:38 This is quite interesting, and looks good for the most part, I just wonder if there can be an easier way to get the icon to update

I think we could add some sort of cache to EditorFileSystem that would track all textures imported with those flags and invalidate their cache if the relevant editor settings change. But I also think we can do it afterwards, as an enhancement. I'd like this to be included in the next alpha, before we're completely locked into beta.

I can still fix the comment from @akien-mga and replace TOOLS_ENABLED code with a static member that EditorColorMap could set. Module should be loaded by the point the editor theme is created, because the module is required for it to load resources. So that should work fine regarding the order of operation.

[YuriSizov]

@akien-mga I've pushed an attempt at making it static without a direct editor dependency as a separate commit. Let me know if this is better, then I can squash.

And I hope at this point we'd be okay merging. 👀 I can work on improving this with a better, automatic reimporting on settings change later.

[fire]

I am not able to do a proper review. Can defer to others.

[akien-mga]

Looks good 👍

[akien-mga]

Thanks!