alsa: Add support for mixer element indexes

Some drivers register multiple mixer elements with the same name.
Make the plugin aware of mixer element indexes to tell such mixer
elements apart.

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Tested on soundcard with two "PCM" mixer elements.

Verified to work:
- switching between elements with same name
- switching between elements with different names
- controlling elements with unique name
- saving and restoring configuration
- reading v3.4 configuration (defaults to the first mixer element matching old configuration)