Add minimum fields option

[samboylett]

Adds the option to set a minimum number of fields. Will make the delete icon/button disappear if the number of fields is at or below the minimum.

Above minimum:

At/below minimum:

Also made it so the ordering icons aren't visible unless there are at least 2 layouts present:

And with a little refactoring moved some reactive data to computed properties

[voidgraphics]

Hi @samboylett, thanks for the PR. It would be interesting to also add a php validation rule automatically when setting a minimum number of rows. What do you think?

[riptin]

Please resolve this!