Fixed warnings in asset package
Fixed most of the warnings in audio package

* Mostly trivial changes
* Roughly 100 warnings in total
* Mostly raw type warnings
* Missing override
* Hide inherited field (playing)
* Some casts were necessary
* And several per-method template names
* Covariant return types were used where appropriate
Use StaticSound for sounds and StreamingSound for music assets
Made BaseSoundSource fields private
Added Static/StreamingSound to serialization libs
Removed BaseSoundSource playing member
Fix type names with underscores
Make AssetData conversions in AssetManager more explicit
Fix checkstyle type name warning
Merge pull request #1073 from msteiger/warnings

Fixes most eclipse warnings in audio and asset packages