Smarter testing if a view can be tapped

The current Frank code tests if the touch point is inside the window bounds. I have modified the code to:

1. Check whether user interaction is not disabled for the application (`[UIApplication isIgnoringInteractionEvents]`, common case when switching view controllers)
2. Check whether the view can be actually touched (not overlapped by other views) using `-[UIView hitTest:withEvent:]` method

This is possibly connected to Issue #163 - even if we can't check whether a view is visible, we can check if it can be touched.