Top Toolbar: Refine icons and handle document title

[scruffian]

When the top toolbar option is selected and you select a block, the document title in the site editor awkwardly overlaid. It should be hidden when a block is selected:

Note also that in this mode to enable the extra space necessary, the mockups here hide the Tools selector.

Additionally, the expand/collapse icons have been updated to double-chevrons, where we can use these:

---

Initial issue

What problem does this address?

With the changes in #40450, the top toolbar can now become very large, to the point where it overlaps the command center:

What is your proposed solution?

One option worth exploring is having items moving into/out of the ellipsis menu in the toolbar, as more/less space is available. This would require nested toolbars:

[annezazu]

Adding this to the triage board of 6.3 for consideration.