Very minor changes for the upcoming commit
Added audio mixer track UI component and EQ.

Summarized list:
- Simplified libawe's filtering interface for operations on entire buffers.
- Added IIR filter for libawe with coefficient generators for LPF and HPF.
- Added Metering filter for libawe which collects audio stats from a buffer.
- Modified libawe's 3-band equalizer to use the new IIR filter.
- Added AudioTrack UI component.
- Added ClanLib's GUI resources as required by AudioTrack.