Emit UI event on zoom control clicks. (#4025)
Emit UI event on zoom control clicks. (#4025)

* Emit UI event on zoom control clicks.

* Fix typo

* Update jsdoc for mouse event handlers.
rachel-fenichel/blockly,rachel-fenichel/blockly,google/blockly,google/blockly,mark-friedman/blockly,rachel-fenichel/blockly,mark-friedman/blockly,google/blockly,google/blockly,rachel-fenichel/blockly,mark-friedman/blockly,rachel-fenichel/blockly,google/blockly,rachel-fenichel/blockly,mark-friedman/blockly,google/blockly,google/blockly,mark-friedman/blockly,mark-friedman/blockly,rachel-fenichel/blockly