alsamixer: Replaced hardcoded keys by binding arrays

alsamixer: Replaced hardcoded keys by binding arrays

This commit is a preparation for making keybindings configurable.

The hardcoded keys in the switch statements of mixer_widget.c,
textbox.c, proc_files.c and card_select.c have been replaced by
command enums. The command that shall be executed is looked up in
the arrays `mixer_bindings` and `textbox_bindings`.

The channel_mask enum has been made public, since LEFT and RIGHT are
used as command arguments in bindings.c.

Signed-off-by: Benjamin Abendroth <braph93@gmx.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>