alsamixer: Use a struct for storing color attributes

alsamixer: Use a struct for storing color attributes

This commit is a preparation for the configuration file parser.

The `int attr_*` variables have been moved into a separate struct.
Members of that struct are alphabetically ordered, so an attribute can
later be accessed by its name.

Added `int get_color_pair(short fg, short bg)` for returning or
creating a color pair number.

Added call to `use_default_colors()` for enabling access to the
terminal's default color (-1).

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