Proposal for supporting wide widget controls

When a wide widget control is collapsed, the triangle arrow that normally points downward instead points rightward:

> ![customize_twenty_thirteen__wordpress](https://f.cloud.github.com/assets/134745/2108574/6a59f176-8fe1-11e3-9e7a-8dedd853b60d.png)

When a wide widget control is expanded, the arrow switches to be leftward pointing, to indicate the direction it will collapse.

> ![customize_twenty_thirteen__wordpress-9](https://f.cloud.github.com/assets/134745/2108651/40dcde66-8fe2-11e3-9872-406b9cf0ba1e.png)

When you focus on a wide widget control, the corresponding `widget-top` will highlight to indicate which widget it is part of:

> ![customize_twenty_thirteen__wordpress](https://f.cloud.github.com/assets/134745/2108677/acd3416e-8fe2-11e3-966b-b9fbf4cb965a.png)

If there are multiple wide widget controls open at a time, hovering over the control or the widget title will indicate the two are connected:

> ![customize_twenty_thirteen__wordpress-4](https://f.cloud.github.com/assets/134745/2108683/cafc89b6-8fe2-11e3-94b3-3bff749d8f84.png)

Todos:

- [x] Ensure that newly-added wide widgets also properly initialize
- [x] Ensure that focused/hovered wide widget control has the higher `z-index`
- [x] Stop allowing multiple controls to be open at a time?
- [x] Position wide widget control vertically along with corresponding widget-top
- [ ] Allow wide widget control to be dragged to another location over the preview?
- [ ] Allow widget control to be made transparent to see through it?

Fixes #18