Block IDs can repeat across multiple uses

[BenHenning]

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Description

Block IDs are not guaranteed to be re-generated when inserting a block from the flyout, and can even be duplicated in other contexts (such as with the minimap plugin--see RaspberryPiFoundation/blockly-samples#2512).

Note that this issue here is not that IDs are reused, it's that the focus system can have multiple nodes with identical IDs. This can result in unpredictable or outright incorrect behavior (see RaspberryPiFoundation/blockly-keyboard-experimentation#521 for one example of what can happen when IDs are duplicated).

The fix here isn't to change core behavior but, rather, to change what the focus system relies upon.

Reproduction steps

1. Open the simple test playground for the latest version of v12.
2. Add a block from the flyout.
3. Observe that the block's ID in the workspace is the same as it was in the flyout.

Stack trace

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[BenHenning]

Note that it would be difficult to explain all the possible failure scenarios that can occur due to this state, so the goal is mainly to fix it, verify that it's fixed, and check the current known failing case (RaspberryPiFoundation/blockly-samples#2512) for behavior differences.

[BenHenning]

Fixed in #9045.