Initial toolbox re-theming

Changes to the toolbox's theme, based on [shorlander's mockup](people.mozilla.com/~shorlander/files/devTools-toolbox-i02/tab-devTools-indicator.png). This PR only changes the toolbox chrome, not the tool panels, that's for a different patch.

Things different from the mockup:

1) No separator at the end of tabs. This was problematic due to our use of the `ordinal` attribute to re-order display of tabs.

2) Separators surrounding selected tab don't change appearance.

3) No separation of toggle-able buttons from creation ones (like scratchpad).

4) No cool sliding animation when moving the triangle tab indicator.