Bring consistency to block toolbar for text blocks

[gziolo]

Description

Closes #15096.

Right now the toolbar across text blocks change a fair bit, and in the case of the heading block, it's not possible to set the alignment unless you open the sidebar. Let's simplify and unify that:

A toolbar for text-blocks consists of these elements and in this order:

1. The block switcher. This doubles as a block type indicator, even when a block has no transformations.
2. An optional "level selector". For headings, this lets you select the level of the heading. For the list block, this lets you pick between the type of list.
3. Text alignments. This one can collapse.
4. Inline formatting. Don't put any buttons here that apply on the block level, it's inline-only.
5. More menu.

A few other details in this mockup:

• The More menu button has been made thinner, as the vertical icon affords this.
• The dropdown triangle buttons have been made thinner as well.

How has this been tested?

Screenshots

It's still work in progress. I just wanted to document where we are at the moment.

Types of changes

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.

[gziolo]

@draganescu is working on moving text alignment to the block toolbar in #16682.

I will take care of adding text alignment to the List block in new PR. I'm closing this one for now.

[gziolo]

I opened #16792 as a replacement.