This is the bulk of the work for introducing the central logical unit
for managing and sychronizing focus as a first-class Blockly concept
with that of DOM focus.
There's a lot to do yet, including:
- Ensuring clicks within Blockly's scope correctly sync back to focus
changes.
- Adding support for, and testing, cases when focus is lost from all
registered trees.
- Testing nested tree propagation.
- Testing the traverser utility class.
- Adding implementations for IFocusableTree and IFocusableNode
throughout Blockly.
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