[rbp] Add Pi audio sink for ActiveAE

This removes the existing stub PiAudio engine, and adds a Pi audio sink to ActiveAE engine.

This adds support for GUI sounds.

It switches to paplayer as the default audio player. This enables missing features like gapless, crossfade and replaygain which weren't supported by omxplayer.

There is a small performance cost to using ActiveAE. e.g. streaming silence costs about 2% of CPU.
The cost should be zero when guisounds and streamsilence are disabled.

paplayer performance seems acceptable.

resampling of gui sounds is quite expensive (~200ms per sound) and is typically required the first time a sound is played after changing the sink's sample rate. It doesn't typically cause a problem, but adding GPU acceleration for this may be beneficial in the future.

The Pi audio sink has been present in milhouse and miappa test builds of openelec and raspbmc for the last few weeks without significiant problems reported.