patch 8.1.1513: all popup functionality is in functions, except :popupclear

Problem:    All popup functionality is in functions, except :popupclear.
Solution:   Add popup_clear() for consistency.  Also rename sound_stopall() to
            sound_clear().
diff --git a/src/sound.c b/src/sound.c
index ac6c377..e28ccff 100644
--- a/src/sound.c
+++ b/src/sound.c
@@ -156,12 +156,18 @@
     sound_play_common(argvars, rettv, FALSE);
 }
 
+/*
+ * implementation of sound_playfile({path} [, {callback}])
+ */
     void
 f_sound_playfile(typval_T *argvars, typval_T *rettv)
 {
     sound_play_common(argvars, rettv, TRUE);
 }
 
+/*
+ * implementation of sound_stop({id})
+ */
     void
 f_sound_stop(typval_T *argvars, typval_T *rettv UNUSED)
 {
@@ -169,8 +175,11 @@
 	ca_context_cancel(context, tv_get_number(&argvars[0]));
 }
 
+/*
+ * implementation of sound_clear()
+ */
     void
-f_sound_stopall(typval_T *argvars UNUSED, typval_T *rettv UNUSED)
+f_sound_clear(typval_T *argvars UNUSED, typval_T *rettv UNUSED)
 {
     if (context != NULL)
     {