Selection mode: add icon before the label

[jasmussen]

Extracting this individual task from #18667.

There is a visual connection between the block toolbar when you're editing, and the block toolbar when you are selecting. Here's the mockup for editing a cover block:

Here, that cover is "selected":

Here's what's currently shipping:

We should add icons to the selection mode toolbar and place them precisely so they stay in the same place when you are switching back and forth between selection and edit modes.

[jasmussen]

Hooray! Fixed!

I spotted in that GIF a discrepancy in the drag handle positioning but I'll fix that separately.