total refactoring of the texture restoration process on a context loss

You can now restore a texture using a "onRestore" callback function. The default behaviour is still to keep the original bitmap data around for the restoration, but you're able to override this by creating your own restoration mechanism -- which might reload the file from disk, recreate it from an Embedded Bitmap, etc.
using 'onRestore' functionality from AssetManager; small 'uploadData' adjustments

The AssetManager now loads the textures from the original sources in case of a context loss. It can thus restore all textures without having to cache any bitmap data.
using 'onRestore' functionality in TextField (on TTF rendering)

Thus, text fields no longer need to cache their bitmap data for a context loss.
added 'Texture.fromEmbeddedAsset' method and using it in samples
added GC hints after AssetManager loading