Support for pause/resume of game rendering loop (#476)

* added pause/resume

* don't override displaymode in game engine

* avoid setting light sensing display mode

libs/core/shims.d.ts

@@ -509,6 +509,12 @@ declare namespace led {
     //% parts="ledmatrix" advanced=true shim=led::setDisplayMode
     function setDisplayMode(mode: DisplayMode): void;
 
+    /**
+     * Gets the current display mode
+     */
+    //% weight=1 parts="ledmatrix" advanced=true shim=led::displayMode
+    function displayMode(): DisplayMode;
+
     /**
      * Turns on or off the display
      */