Add FileHelpers.ResolveRelativePath and use it.

Fixes #2577
Simplify and fix for WINRT
Implemented GraphicsAdapter.IsWideScreen
Added GraphicsAdapter.IsWideScreen documentation
Merge pull request #2597 from thefiddler/widescreenfix

Implemented GraphicsAdapter.IsWideScreen
Merge pull request #2585 from danzel/combine-uri-file

Add FileHelpers.ResolveRelativePath and use it.
Merge branch 'develop' of http://github.com/mono/MonoGame into develop