Platform: ability to access particular Application subclass from Screen.

This isn't exactly encouraging good practices, but I didn't come up with
better solution yet.
SceneGraph: renamed Animable::group() to Animable::animables().

Fixes ambiguous case when inheriting multiple grouped features in a
single object. The original functions are now aliases to the new ones,
are marked as deprecated and will be removed in future release.
SceneGraph: add Drawable::drawables() function.

Similar to Animable::animables().