Basic WinForms support

[rzhw]

Adds a PlatformBitmapLoader. Since it doesn't seem possible to tell if an app is meant to be WinForms or WPF, and the two can be mixed, it's namespaced under Splat.WinForms and would need to be manually set as the current bitmap loader.

[anaisbetts]

Merged, thanks @rzhw! My only change was, you should just omit the private keyword, it isn't necessary (at least for code that's written like Splat and RxUI, in the One True C# Style™)