[FrogScroller] fix NPE. the first "/" means root folder. But we want relative folder

Xamarin.iOS 8.6.0.12
[FrogScroller] introduce property for code simplification

Xamarin.iOS 8.6.0.12
[FrogScroller] fix bug with abs img path. Change the way to build path to file

Xamarin.iOS 8.6.0.12
[FrogScroller] move properties to the top of the class declaration

Xamarin.iOS 8.6.0.12
[FrogScroller] move ImageDetails class declaration to separate file
[FrogScroller] TILE_IMAGES define is for nothing, because TILE_IMAGES if defines always

Xamarin.iOS 8.6.0.12