Restyle scrollbars

Two kinds of scrollbars:
* Light for most UI surfaces
* Dark for the editor to match the theme and not distract as much

@cleercode, what do you think? This has been a point of some contention, see #2406.

I could adjust the colors, thickness and anything else, obviously. But I think that the system scrollbars just don't look good, especially on Mac. They also look different on different platforms: it's the only remaining UI element that still does; the rest look the same everywhere.

![screen shot 2014-06-10 at 5 09 47 pm](https://cloud.githubusercontent.com/assets/5606182/3238720/bef9e232-f0fc-11e3-894c-35b8e64ec35d.png)

Also note that the Outline toggle button and the outline positioning will presumably change with #2391, so it will overall look nicer.