Add ThemeDB, expose previously static Theme methods

[YuriSizov]

This PR adds a ThemeDB engine singleton that can be used, so far, to access the default and the project theme, as well as fetch (and set!) fallback values for the entire theming system.

This aims to provide ground work for any future implementation of godotengine/godot-proposals#4486. Regardless of what we agree upon in details, a theme database singleton makes a lot of sense to provide access to the theming system without overburdening the Theme resource itself with even more global data. I don't think I can finalize the idea of godotengine/godot-proposals#4486 before the Beta, let alone get it approved. So this should be a good first step to lay foundation for future changes. I expect ThemeDB to host and give access to well-defined theme properties of controls.

But thus far it only exposes static information related to themes and takes a bit of code off of theme.cpp's hands.

[akien-mga]

I would expect this under gui more than its own folder, especially as it's only relevant for gui (with the exception of the GUI-adjacent Window and Label3D).

[YuriSizov]

Yeah, and Theme is a resource, so it could also be under resource... 🙃 I have a plan for another class that could go into scene/theme, and we already have audio that similarly has shared classes in their own folder.

[YuriSizov]

This PR will keep failing CI because the new singleton needs to be added to the regression testing project. Currently it doesn't know about ThemeDB, creates an instance of it and then frees it, thus breaking the singleton.

All other singleton exceptions are on this list (or are excepted based on the word "server" in their name, ThemeServer would've worked like a charm 🙃):
https://github.com/godotengine/regression-test-project/blob/4.0/CreatingAllThings/CreatingAllThings.gd#L4

Obviously, there is no point updating the test project if this PR is not approved. So the plan is to get this reviewed and approved, then update the test project, then rerun the regression test here, and then it can be safely merged.

[akien-mga]

Thanks!