Add methods to remove theme overrides

[KoBeWi]

Resolves #32110

[akien-mga]

Aside from add_theme_color_override, all other theme property overrides can be cleared by passing a given value (0 or null) as written in the docs.

If we add remove_theme_*_override for all of them, then maybe we should also remove the behavior where they clear themselves on specific values.

[YuriSizov]

I say we remove that behavior in master, but keep it in 3.x (as this PR can be cherrypicked otherwise). It doesn't make sense for a function called add_override to have a special case value that removes an override instead. I feel like it's just a hack for the Inspector, and nothing else (which should be tested if we do remove this trick).

[KoBeWi]

If we add remove_theme_*_override for all of them, then maybe we should also remove the behavior where they clear themselves on specific values.

I tried this. When you set a null stylebox it will crash, so I just left it as is.

[akien-mga]

When you set a null stylebox it will crash, so I just left it as is.

Sounds like a good reason not to leave it as is :) Or at least this needs a bug report as it contradicts the documentation.

[KoBeWi]

Maybe it should just throw error on null 🤔
btw by "as is" I mean the current behavior of removing override.

EDIT:
Also other add_*_overrides don't seem to have code that would handle erasing...

[KoBeWi]

Ok altered the old behavior in another commit.

[akien-mga]

Thanks!