stop simulator while compiling (#848)
stop simulator while compiling (#848)

* stop simulator while compiling to native

* track compilation state

* deal with multiple clicks on compile

switch-education/pxt,Microsoft/pxt,playi/pxt,switchinnovations/pxt,switch-education/pxt,Microsoft/pxt,playi/pxt,Microsoft/pxt,playi/pxt,switchinnovations/pxt,switch-education/pxt,playi/pxt,Microsoft/pxt,switchinnovations/pxt,switch-education/pxt,switchinnovations/pxt,Microsoft/pxt,switch-education/pxt