Fix locking in waitForEvent() and sample playing

libs/core/linux.cpp

@@ -269,11 +269,13 @@ void waitForEvent(int source, int value) {
             pthread_mutex_lock(&eventMutex);
             t->waitSource = source;
             t->waitValue = value;
+            stopUser();
             // spourious wake ups may occur they say
             while (t->waitSource) {
                 pthread_cond_wait(&t->waitCond, &eventMutex);
             }
             pthread_mutex_unlock(&eventMutex);
+            startUser();
             return;
         }
     }

libs/core/pxt.h

@@ -7,6 +7,8 @@ namespace pxt {
 void raiseEvent(int id, int event);
 int allocateNotifyEvent();
 void sleep_core_us(uint64_t us);
+void startUser();
+void stopUser();
 
 class Button;
 typedef Button *Button_;