Extract StyleBoxFlat, StyleBoxTexture and StyleBoxLine in their own file

[Geometror]

Part of the ongoing effort to have only one class per file with the goal of improving the structure of the codebase in terms of readability and compilation speed.
style_box.cpp contained already over 1000 LOC, which made navigation quite cumbersome.
In addition, some methods and members have been reordered to further improve clarity.

[Mickeon]

I agree with this greatly, but where has the "ongoing effort" been stated, if anywhere?

A lot of classes could benefit from it, absolutely, but it makes me wonder if it would be worth it to group them in sub-folders someday and what is the argument against it.

[Geometror]

It came up on RC a few times and in the discussion around #33027 as well, but like some other things we discussed it was never written down formally in https://docs.godotengine.org/en/latest/community/contributing/code_style_guidelines.html. We still need to do that.
Grouping some header/source files together in a sub-folder could make sense, but one (minor) argument against this would be that having a deeper folder structure impedes navigation (especially in tree views).

[akien-mga]

I approved the change in principle but I'm not sure this is the right time to merge such changes, as there's a lot more that would need splitting if we decide that this is the new standard. Started a discussion in maintainer channel, we can formalize this in a proposal later on to document the direction we want to go for. For now I'd tend to keep this and similar changes for 4.1.

[YuriSizov]

With Akien's comment in mind and the timing, I've kicked this to 4.x (realistically to 4.1) for now.

[YuriSizov]

@Geometror I think we are ready to merge it in the next couple of weeks. So if you could do a rebase, it would be great!

[YuriSizov]

Mostly good, some changes are still required.

[YuriSizov]

Thanks!