Unbind event handlers in dispose of zoom controls. (#4031)
Unbind event handlers in dispose of zoom controls. (#4031)

* Unbind event handlers in dispose of zoom controls.

* Move event handler references to constructor.
google/blockly,google/blockly,rachel-fenichel/blockly,google/blockly,mark-friedman/blockly,mark-friedman/blockly,google/blockly,google/blockly,rachel-fenichel/blockly,rachel-fenichel/blockly,mark-friedman/blockly,google/blockly,mark-friedman/blockly,mark-friedman/blockly,rachel-fenichel/blockly,mark-friedman/blockly,google/blockly,rachel-fenichel/blockly,rachel-fenichel/blockly,rachel-fenichel/blockly