* Implement Blockly.Events.filter in linear time
For large App Inventor projects (order 1k+ blocks, 100+ top-level
blocks), the O(n^2) behavior of Blockly.Event.filter was causing
performance issues when rearranging blocks or pasting from the
backpack. This commit provides a linear merge implementation using a
key that uniquely identifies a block so that multiple events targeting
the same block are merged. This change benefits from O(1) amortized
lookup using an object as a key-value store.
* Add event filter unit tests and fix logic bugs
* Update Blockly.Events.filter unit tests