Add conversion for common Theme Overrides

[Maran23]

Fixes: #66052
This PR adds conversion for the most commonly used theme overrides.
This includes:

• Styles (StyleBoxes)
• Icons
• Font colors
• Spacing, Separation, Offsets

-> Margins, StyleBoxes etc. will still work after the migration.

[YuriSizov]

Can't we just rename the sections instead of listing some specific and arbitrary properties?

[Maran23]

Can't we just rename the sections instead of listing some specific and arbitrary properties?

Unfortunately not in all cases.
Sometimes it is as simple as:
custom_styles/normal -> theme_override_styles/normal
and sometimes more tricky:
custom_colors/font_outline_modulate -> theme_override_colors/font_outline_color

And using e.g. custom_styles/ in front of the properties will make sure we really just rename the Theme Override property and not just an arbitrary named variable.

[YuriSizov]

What I mean is, can't we rename custom_*/ to theme_override_*/, before doing specific renames?

[Maran23]

What I mean is, can't we rename custom_*/ to theme_override_*/, before doing specific renames?

Ahh. Yeah, that should be possible. Then the theme conversion needs two steps:

1. Replace custom_*/ with theme_override_*/
2. Rename the properties which were renamed in Godot 4

I can check that out tomorrow. :)

[Maran23]

What I mean is, can't we rename custom_*/ to theme_override_*/, before doing specific renames?

Done. I still opted for the safer approach, so we really won't be converting any unrelated properties/variables.

[YuriSizov]

Thanks!

[YuriSizov]

Cherry-picked for 4.0.1.