Various fixes (#548)

* adding running time micros

* support for system micro seconds

* restore display mode after plotting sprites

* bump pxt

libs/core/input.cpp

@@ -339,13 +339,23 @@ namespace input {
     /**
      * Gets the number of milliseconds elapsed since power on.
      */
-    //% help=input/running-time weight=50
+    //% help=input/running-time weight=50 blockGap=8
     //% blockId=device_get_running_time block="running time (ms)"
     //% advanced=true
     int runningTime() {
         return system_timer_current_time();
     }
 
+    /**
+     * Gets the number of microseconds elapsed since power on.
+     */
+    //% help=input/running-time-micros weight=49
+    //% blockId=device_get_running_time_micros block="running time (micros)"
+    //% advanced=true
+    int runningTimeMicros() {
+        return system_timer_current_time_us();
+    }
+
     /**
      * Obsolete, compass calibration is automatic.
      */