SoundSystem: loading sound resources with our own ResourceHandler.

To load the sound files into the SoundSystem, now we use our own resources
managing system, instead of depending directly from Ogre::ResourceManager.
The "loadSound" functions from SoundHandler and SoundManager
now take a reference to a (properly configured) rrh::ResourceHandler.
Also, minor functional was added to rrh::ResourceGroup and rrh::ResourceHandler.