Various fixes (#548)

* adding running time micros

* support for system micro seconds

* restore display mode after plotting sprites

* bump pxt

sim/state/misc.ts

@@ -86,6 +86,10 @@ namespace pxsim.input {
         return runtime.runningTime();
     }
 
+    export function runningTimeMicros(): number {
+        return runtime.runningTimeUs();
+    }
+
     export function calibrateCompass() {
         // device calibrates...
     }