4 new methods for GridView

4 new methods for StickyGridHeadersGridView.

I'm writing image gallery using your library with "preload image" feature. This means, that when user scroll down gallery, I'm preloading next few pictures to memory, so when user scrolls gallery again images load immediately and everything looks very smooth and cool. On scroll idle I need to know first and last gallery item so I can get next and previous images from my adapter and preload them to memory. That's about "getFirstVisibleAdapterPosition" and "getLastVisibleAdapterPosition".

Now about "get[First | Last]VisibleContentViewPosition". Every item in my gallery has label with creation time on it like "An hour ago", "3 minutes ago" and so on. When activity is showed theese labels shortly becomes outdated so I need to iterate through visible content items (only content, no headers or fillers) and refresh label values on it.