refactor: Update the flyout to account for flyout API changes in core

[gonfunko]

This PR updates the flyout and associated infrastructure for compatibility with the changes introduced in core Blockly in google/blockly#8601. Beyond strict compatibility, it also:

• Registers a block inflater that supports block recycling
• Uses icons/bubbles for blocks-with-checkboxes in the flyout so that the flyout no longer needs to be aware of or manage these. This incidentally fixes There is too much space between blocks and checkboxes in the flyout #200
• Registers an inflater for flyout section headers with status indicators, and a corresponding class to inflate