Make Group an interface

[wesen]

I want to experiment with different Group layouts (for example as rows, as many of my forms are short fields, but many fields). Currently, I can't really do too much because I have to use the Group struct. I would like to play with the group titles and such as well.

Turning Group into an interface would make it fairly easy to experiment more (like with Field). This would also allow pulling in thirdparty huh-improvements easier.

I'm happy to do the necessary work, but wanted to check in before opening a pull request.

[maaslalani]

We'd be open to supporting different layouts such as horizontal layouts. We can likely introduce an option on a Group called WithLayout which perhaps can take LayoutHorizontal and LayoutVertical and displays the view accordingly.