The build in keyboard is enabled by default. When USBDeviceManager

handles keyboard plugged in/out event, it also reset the kbStatus.
Hide the enable/disable keyboard menu item when there is no external
keyboard.

hi. This commit is a workaround for build in keyboard status detection. We first assume the build kb is enabled by default, we can reset the kbStatus when there's keyboard plugged in/out event.

This commit also hide the enable/disable menu when there is no external keyboard. I wanted to disable the menu, but the api is only available by 10.10.

I think this app has something more to polish. Hope we can get a better version.