1. add pthread for every button event.
2. add pthread_mutex_t for every button event, only one can be executing at the time being.
3. add pthread_mutex_t for data handler, not completed yet.

TODO:
write functions, button event for landhere
write functions, button event for kill quadcopters
Merge branch 'thread_GUI' of github.com:QawsQAER/paparazzi into thread_GUI