Use Haxe logo in the HaxeFile.png of the ProjectManager

Granted, I'm no graphics designer, and it's hard to make the haxe logo look good at that scale / recognizable, but I think it's more appropriate to use the haxe logo here. Seems like the old icon for .hx files had a recolored version of the flash logo, which might lead to people getting a wrong idea about haxe.