Improve editor toolbar toggle buttons accessibility

[afercia]

As noted in #558, the buttons in the inline toolbars behave as a toggle. Not just the alignment ones, also bold, italic, etc. and all the ones that need to be toggled to change a state.

Visually, their state is communicated with a color inversion. Semantically, there's nothing to communicate their state. This kind of toggle buttons need an aria-pressed attribute with true/false value. This way, assistive technologies can communicate the buttons state.

As they work now:

Compare for example to what TinyMCE does:

TinyMCE uses divs for the buttons and then "resets" the real buttons semantic with role=ppresentation, but that's not the point, the necessary semantics is all there on the divs.

The editor toolbar would need further a11y improvements that should go in separate issues:

• first of all, a simple, easily discoverable way to focus them using only the keyboard 🙂
• arrow key navigation as currently discussed for the inserter, see Add arrow key navigation to the inserter window. #515
• to discuss with the a11y team: maybe ARIA roles

[afercia]

• a simple, easily discoverable way to focus them using only the keyboard: see Pressing cmd/ctrl as a way to navigate into block toolbars #552
• arrow key navigation: see Add arrow navigation to the editor toolbars #631
• to discuss: maybe ARIA roles: see Should the editor toolbar have a role=toolbar? #632