feat: form and group dedicated styles

[nervo]

This pr follows this one: #257

Instead of having only one style for form and groups in the theme, we introduce two new structs FormStyles and GroupStyles on the same model as FieldStyles. Both of them having a Base style applied to the View.

Please note that ErrorMessage have also been moved from FieldStyles to GroupStyles, as its only used in group view, and in a way that suggests there was a kind of mistake :)

Note also that since both Form and Group theme struct fields are unused today, we may consider that it's not a breaking change.

[maaslalani]

@nervo, I think we shouldn't break the Theme API unless we have a super compelling reason to. I propose we simply start using the Form and Group styles in this PR instead of changing them to FormStyles and GroupStyles

[maaslalani]

If we need Error / ErrorMessage style we can add that as a separate field on the theme.

[nervo]

@maaslalani using Form and Group styles is exactly (and only) the scope of this pr: #257 (which i just rebased)

If you're ok with it, please, consider validating and merging it :)

After that we can discuss the case of the error style on this thread.