ResolutionFileResolver supports DP and have different matching strategy

Additions to [ResolutionFileResolver](https://github.com/libgdx/libgdx/blob/master/gdx/src/com/badlogic/gdx/assets/loaders/resolvers/ResolutionFileResolver.java) see issue #2161

now supports
+ bestMatch: closest size to target screen
+ higherMatch: smallest size larger than the target screen
+ lowerMatch: largest size smaller than the target screen

Also added Density pixel resolution metric, match sizes based on screen density.