Remove Gutenberg CSS classes and adjust CSS accordingly.

[afercia]

Summary

This PR can be summarized in the following changelog entry:

• Fix the buttons position in the structured data blocks.

Relevant technical choices:

• Gutenberg moved some components from editor to block-editor
• as a consequence, also some Gutenberg CSS classes have now different names e.g. editor-inserter__toggle became block-editor-inserter__toggle
• we shouldn't use Gutenberg CSS class names in our components to start with, as they're not part of a formalized API (or so they dixit)
• this PR removes any usage of editor-inserter__toggle
• removes components-icon-button from the JS, though it's still used in our stylesheet
• changes the "Add total time" Button component to IconButton: this was a hack made necessary by the IconButton lack of support for refs. So we used a Button and manually added the CSS class components-icon-button. Since Make IconButton able to be referenced. WordPress/gutenberg#14163, IconButton now is able to get a ref thus we can use an IconButton

Test instructions

• activate the Gutenberg plugin, at the moment latest master is 5.3.0 WordPress/gutenberg@95642a4
• create a post
• add a How-to block with some steps
• add a FAQ block with some QA
• check all the buttons are properly positioned
  • How-to: Add total time
  • both blocks: Add Image, Delete, Insert, and the "Add step / question" at the bottom
• press the How-to "Add total time" button using the keyboard
• tab to the "Delete total time" trash icon button and press it
• verify focus is moved back to the "Add total time" button

UI changes

• This PR changes the UI in the plugin. I have added the 'UI change' label to this PR.

Fixes #12496

[hwinne]

CR Done 👍

[johannadevos]

Acceptance 👍