Inserter menu ARIA roles and attributes

[afercia]

Splitting this out from #1823

The inserter is now in a stable enough state but still uses ARIA roles and attributes that where added in a first implementation, when the block types were presented in a sort of vertical list.

Testing with screen readers, it appears there's the need to make some adjustments. I think the roles menu and menuitem can stay, but there's probably the need to "flatten" the ARIA tree and use some role="presentation" on other elements. This would require some testing.

Nooticed so far:

• JAWS 2018 and IE11 don't announce the name of the block types. They announce just "menu"
• Firefox and NVDA incorrectly report the number of items in a category group (right now, each group is an ARIA menu), see screenshots below

Seems there's no consistency in the (wrong) number of items that gets announced:

in this case, there are 3 items but then NVDA announces "n of 4"

and in this case, there are 3 items but then NVDA announces "n of 7"

[afercia]

The Inserter has been completely refactored and it’s now based on accordions. Also the ARIA part has been greatly simplified in #7058. Closing.