alsamixer: added mouse support

alsamixer: added mouse support

Mouse support has been added for mixer_widget.c, card_select.c and
proc_files.c.

In the mixer widget the mouse is handled as follows:
- After an element has been printed in mixer_display.c, a call to
  clickable_set() will store the coordinates of the drawn area plus the
  command enum that should be executed on click. An optional argument
  holds an index which points to the selected mixer control.
- on_mouse_click() searches for a matching rectangle, focuses the mixer
  control and returns the command enum.

In the menu widgets, the menu_driver() function handles mouse input.

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