Show widget form controls initially in collapsed state

[westonruter]

Since all widget form controls are shown in full when the customizer section is expanded, it can take up a lot of room and make it difficult to navigate to the desired widget. So they should all be presented in a minimized state, though not like an accordion since we want to be able to have multiple widget controls open at a time.

[johnregan3]

@westonruter Could this be an accordion that allows you to have multiple panels open?

[westonruter]

@johnregan3 sure! Is that an option with the accordion library used in core? I was thinking it might be nice to replicate this UI:

[johnregan3]

@westonruter That looks good. I always try to use WP UI. I'd like to take a stab at this one.

[westonruter]

@johnregan3 go for it!

[johnregan3]

@westonruter I'm still working out how to add a PR to an issue via hub. In the meantime, you can review my PR at #12