Added pause-on-active option

Windows only for now. When set to true, pauses mining when user touches mouse or keyboard.

src/base/kernel/Platform.h

@@ -49,6 +49,7 @@ public:
     static inline const String &userAgent() { return m_userAgent; }
 
     static bool isOnBatteryPower();
+    static bool isUserActive();
 
 private:
     static char *createUserAgent();