GraphX v1.0.0 nullsafety preview.

[1.0.0-nullsafety.0]

• initial migration to null-safety
• fix non-working examples.
• fix a bug with GText layout.

[0.9.9]

• major fix for GTween when running lots of SceneControllers instances.
• fix stage dispose exception with keyboard/pointer.
• added some more ColorFilters.

[0.9.8]

• major code refactoring, remove svg_flutter as dependency to avoid breaking changes.
• changed all methods that takes int hex colors + alphas, to Color (beginFill, beginGradientFill, stage.color, etc).
• fix trace for web and dartpad output.
• renamed most common classes (that do not start with G) to follow a GName pattern.
  • SimpleText > GText
  • GxIcon > GIcon
  • Sprite > GSprite
  • GxPoint > GPoint, and so on...
• added dispose() to KeyboardManager and PointerManager... so it manages hot reloader better.
• added LayoutUtils.objectFit() to resize GDisplayObject (specially Bitmaps), using BoxFit.
• downgraded flutter_svg to 0.18.1 in "example" to make it usable in beta channel.
• refactor resource_loader.dart, changed most methods to use named params, now only stores in cache if a cacheId is defined (and also returns the cached data).
• GText now uses Flutter's TextStyle to make it more convenient.
• added ResourceLoader.loadNetworkTextureSimple() using a simple http call in flutter (useful for dartpad).