Update continuous toolbox plugin for flyout API changes

[gonfunko]

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Component

Continuous Toolbox plugin

Description

The flyout API is changing in Blockly v12 (see google/blockly#8601), and the continuous toolbox plugin will need to be updated for compatibility. Most of the needed changes have already been made in gonfunko/scratch-blocks#209 (review) and will just need to be backported, but we'll need to coordinate merging/publishing to coincide with the v12 release.

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[gonfunko]

ContinuousToolbox.getInitialFlyoutContents_() should also be refactored to allow subclasses to make changes. It should probably call a conversion function for each item, which subclasses could override to support additional types of flyout items or just delegate to super.