Toolbar Styling improvements PR

Several toolbar style improvements:

- Height of the toolbar is now 40px (non-retina).
- Background-color is now white #fff by default, and customizable.
- The spacing between all toolbar icons has been removed.
- The toolbar's border-top and divider color has been changed to #c8c8c8
- The toolbar icons normal color is now #7c7c7c and customizable.
- The toolbar divider line is 28px tall (non-retina), and centered.
- Active items are now tinted #0087be (the line that comes up below the icons too).
- The toolbar now collapses when editing the title, since styles can't be used.
@thianhlu - These are the color codes I couldn't find a match for, in our `WPStyleGuide` class: #7c7c7c, #c8c8c8.  If you agree to it I can ping the users mentioned by @aerych.