[haxe] HaxeFlixel official runtime

[davidetan]

This is a issue to let users receive updates regarding the progress of HaxeFlixel support as an official runtime.

We already have a working version of the runtime that can be found on a dedicated branch.

However, to make it working correctly we had to submit a couple of PR to fix some bug with meshes (1, 2). Without these two PR release, animations using slot color/alpha won't work correctly.
We're waiting for HaxeFlixel next release to release our runtime.

Todo:

• Implementation
• Wait for HaxeFlixel next release and add that version as minimu requirement
• Merge the haxe-flixel
• Documentation
• Release
• Blog Post/News