Themes in Brackets core

Initial commit.  A few items remain to be done, but themes are now fully functional as a core component so you can start testing this out.

https://github.com/adobe/brackets/wiki/Themes

* [x] ThemeManager goes into core.
* [x] ExtensionLoader has special logic for themes extensions that don’t have JS
* [x] Settings pared down to just your “General” screen
* [x] All of the other loading/scanning code can go away
* [x] Use jQuery promises, sorry :*(
* [x] Settings screen should not use Knockout
* [x] Use new prefs API
* [ ] Unit tests for theme loading and the customizations (scrollbars, etc.)
This line shouldn't be necessary because it should already be returning a promise.