Alternative center panel layout: use position left/right rather than
left/right padding. Slightly simpler and possibly (don't know until testing)
may be faster too.
Code review cleanups:
- add "in-appshell" body style, mutually exclusive with "in-browser"
- use symbolic constants for more colors
- rename Resizer's "forcepadding" to "forceleft" to reflect new layout
- remove unneeded qualifier on "#main-toolbar" selector
- remove unused padding on Live Preview icon rule (overrided by other rules)
- improve docs