Improve Windows XP support

[jgranick]

I got a report that Windows XP returns an error, saying that shell32 does not include SHCreateItemFromParsingName

This StackOverflow post lists how to make a Windows XP-compatible replacement:

http://stackoverflow.com/questions/20885556/replacement-for-shcreateitemfromparsingname-on-windows-xp/20887104#20887104

I have tried to apply this update in your code style, it appears to be working here. Thanks!

[jgranick]

We have users who have tested OpenFL titles on Windows XP that included file dialogs, and have heard no complaints. This code definitely does work on Windows 7+, where we've tested more regularly on the core team. Thanks!

[mlabbe]

There is, explicitly, no support for Windows XP in Native File Dialog right now. (See the readme). This is due to the fact that I am not using OpenFileDialog et al., and instead am using a Vista-level API. Some versions of XP support it, and they won't crash, but there is no guarantee.

I would be open to a pull request that implements nfd_win_legacy.cpp which detects the windows version, and calls APIs that are guaranteed to exist on earlier versions of windows. It should be possible to route these at runtime, so every version of windows is served by Native File Dialog without a recompile.