Added mouse drag detection threshold.

Hey!

I've had noticeable CPU issues on an app I'm working on when using INAppStoreWindow. Measuring in Instruments pointed to `[NSWindow setFrameOrigin:]` call within `[INTitlebarContainer mouseDragged:]` as taking ~70% while parent while loop ~30%.

With this commit, the situation is considerably better on CPU load. `setFrameOrigin:` is only called when drag occurs on container view.