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_unix.cpp

@@ -158,3 +158,10 @@ bool xmrig::Platform::isOnBatteryPower()
     }
     return false;
 }
+
+
+bool xmrig::Platform::isUserActive()
+{
+    // TODO
+    return false;
+}