Implemented audio mixing of audio module.
Miscellaneous changes:

* Implemented datablocks for Converter.
* Scenes are by default, inactive and not rendered.
* Enabled physics world step compensation.
* Cursor uses XWarpCursor on Linux platform.
Major restructuring:

* Audio module was moved into its own project.
* FFmpeg wrapper was moved into its own project.
* Addons module was moved into its own project.