Non-visible views and OnScreenSwitchListener

[RedHatter]

When horizontalpager had a child view that had visibility set to View.GONE it would correctly not call onLayout for it, but would incorrectly still take it into account in other calculations. This behavior would cause an empty black screen to appear at the end. My patch fixes this by not counting children with View.GONE set.

My patch also adds onScreenSelected to OnScreenSwitchListener to notify user about a new screen but before the animation.

[ysamlan]

Thanks for the contribution, but this library is obsoleted by Google's release of their own ViewPager library as part of the Android Compatibility Package (since compatlib rev 3). I would recommend using that for any new development; this library is not maintained any longer, and I can't see any advantage to using this over Google's official blessed implementation.

[RedHatter]

Google's ViewPager is several times more complex to use. So if a simple pager is needed for a small part the project works much better.