Changes to Libgdx FileHandle

So far libgdx would return either backslashes or forward slashes for paths such as “/home/badlogic” or “d:\Users\badlogic” depending on the operating system. I changed this long-standing issue (which can affect AssetManager if you aren’t careful). All FileHandle implementations on all backends will now return forward slashes on all systems/backends for all methods that return a string representation of the FileHandle.