Improve animation support in fromLibGdx()

[Descrout]

Get the image index in the atlas, if its not -1, this means the image is a part of an animation. Add _$index suffix to name so it can be used by animation.addByPrefix.

Example

Code:

var frames = FlxAtlasFrames.fromLibGdx(AssetPaths.myAtlas__png, AssetPaths.myAtlas__atlas);
var player = new FlxSprite();
player.frames = frames;
player.animation.addByPrefix("idle", "adventurer-idle", 12);
player.animation.addByPrefix("crouch", "adventurer-crouch", 12);
player.animation.play("idle");

myAtlas.atlas :

myAtlas.png
size: 2048,512
format: RGBA8888
filter: Nearest,Nearest
repeat: none
adventurer-idle
  rotate: false
  xy: 324, 51
  size: 50, 37
  orig: 50, 37
  offset: 0, 0
  index: 2
adventurer-idle
  rotate: false
  xy: 568, 53
  size: 50, 37
  orig: 50, 37
  offset: 0, 0
  index: 0
adventurer-idle
  rotate: false
  xy: 1974, 391
  size: 50, 37
  orig: 50, 37
  offset: 0, 0
  index: 1
adventurer-crouch
  rotate: false
  xy: 1981, 430
  size: 50, 37
  orig: 50, 37
  offset: 0, 0
  index: 2
adventurer-crouch
  rotate: false
  xy: 1870, 391
  size: 50, 37
  orig: 50, 37
  offset: 0, 0
  index: 0
adventurer-crouch
  rotate: false
  xy: 1922, 391
  size: 50, 37
  orig: 50, 37
  offset: 0, 0
  index: 1