fix: update the flyout for compatibility with the new flyout API

[gonfunko]

This PR updates the continuous flyout subclass for compatibility with the new flyout API. Additionally:

• Checkable-block handling is almost entirely removed, since this is now handled by the checkbox icon/bubble. This incidentally fixes There is too much space between blocks and checkboxes in the flyout #200.
• Handling of recycling is also almost entirely removed, since this is now handled by the block flyout inflater subclass.

Several methods have been mostly cribbed from the plugin's implementation with slight changes for API compatibility; ultimately these will be upstreamed into the plugin and should then be removed here in favor of the superclass implementation.

[rachel-fenichel]

LGTM. Please file a bug and add TODO(bugNum) in the appropriate places for the follow-up work you have in mind.

[gonfunko]

Opened #211 and google/blockly-samples#2464 to track and added comments to the relevant methods.