Remove widgets from struct alsa_elem and add data to callbacks

Rather than having widget/widget2/widget_callback fields in the struct
alsa_elem, have a list of callbacks and allow private data to be
passed to callbacks.

src/widget-dual.h

@@ -9,8 +9,9 @@
 // buttons:
 // first button disables/enables the feature
 // second button switches between the two features states
-void make_dual_boolean_alsa_elems(
+GtkWidget *make_dual_boolean_alsa_elems(
   struct alsa_elem *alsa_elem,
+  const char       *label_text,
   const char       *disabled_text_1,
   const char       *enabled_text_1,
   const char       *disabled_text_2,