Add `FileDrag` to the `Input` enum

[kaikalii]

I wanted piston to be able to hand file drag events, so I added an enum called FileDrag which contains variants that mirror the file drag events of glutin. I added this enum as the only field of a new variant of Input, Input::FileDrag. I was going to add it to the Motion enum instead, but file dragging events include a PathBuf, which is not Copy.

[bvssvni]

Merging.

[bvssvni]

Thanks!