added method to get all contained char IDs from bitmap font (closes #554)
added 'hasChars' utility method (closes #347)
now conserving 'onReady' texture option when loading ATF texture (closes #555)
avoiding 'slice' when possible (refs #493)
added 'over' and 'disabled' states to Button (refs #549)
fixed bug introduced with previous AssetManager change

At the same time, this change allows to change the texture settings while filling up the queue.
In the following sample, the first bunch of textures will use mipmaps, the second bunch won't.

    assets.useMipMaps = true;
    assets.enqueue(appDir.resolvePath("textures/with-mipmaps"));
    assets.useMipMaps = false;
    assets.enqueue(appDir.resolvePath("textures/without-mipmaps"));
added 'textureFormat' property to AssetManager; added more ASDoc 'default' infos
optimization in 'DisplayObject.removeEventListeners' (refs #493)
small 'advanceTime' optimization in MovieClip (closes #558)
added 'overlay' sprite to Button class (closes #559)
fixed compiler error (closes #562)
added missing semi-colons and removed dead code
made some methods in AssetManager 'protected' for better extensibility (closes #553)
more reliable 'context' property in shared context situations, if the runtime supports it
now detecting/storing profile string sooner when using a shared context
fixed error in previous commit
small readability changes
fixed handling of invalid image data in AssetLoader (closes #563)
added 'texture' property to BitmapFont class
get characters in bitmap font