mixer: factorize backend emix infra
The mixer gadgets suffer of a bad design between old and new gadget infra.
This commit refactorize the mixer backend to get only one instance
between old and new gadget. This resolve many problems as e_client_volume
integration or the default_sink choosen in one of this modules.
Now it will be easier to maintain this code and better support when we will
remove old gadget infra.
Reviewers: zmike, devilhorns, stephenmhouston
Reviewed By: zmike
Differential Revision: https://phab.enlightenment.org/D7718