Improve appearance of controls in the toolbar, with grouping

[thibaudcolas]

This is a subset of #75, with the goal of reaching feature parity with the current Wagtail editor.

Here is what we want:

Tasks:

• Button styles
• Button grouping / segmented buttons
• More obvious grouping controls into "things of can do to one word" vs "things you can do to a block of words"
• Bigger editor controls and more descriptive icons.
• Naming of labels (non-dev people don't know what H3, BR etc mean.)

The button grouping is harder to solve than it might look – this target screenshot is what the most common editor will look like, but people may also have way less (or way more) of a given type of control, and we still need to ensure the toolbar looks good no matter how much screen real estate is available.

[thibaudcolas]

Addressed in #98.