Support Blockly pinch to zoom gestures (#3682)
Support Blockly pinch to zoom gestures (#3682)

* Support Blockly pinch to zoom gestures
playi/pxt,switchinnovations/pxt,switch-education/pxt,Microsoft/pxt,switchinnovations/pxt,Microsoft/pxt,Microsoft/pxt,playi/pxt,switch-education/pxt,Microsoft/pxt,switchinnovations/pxt,switchinnovations/pxt,switch-education/pxt,switch-education/pxt,switch-education/pxt,playi/pxt,Microsoft/pxt,playi/pxt