big simple mixer update

big simple mixer update

- exported all necessary functions to create a mixer module outside alsa-lib
- separated simple mixer API from the simple mixer implementation
  (using callbacks as usuall)
  - src/mixer/simple.c is the core
  - src/mixer/simple_none.c is the current (no-abstraction) implementation
    based on control names; note that this module does not depend on
    internal ALSA structures now
  - src/mixer/simple_abst.c is the ongoing abstraction which will use
    external dynamic modules; src/conf/smixer.conf will describe which
    modules will be used depending on the components from the driver