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/proto/sound.pro b/src/proto/sound.pro
index 43e4727..cf8439a 100644
--- a/src/proto/sound.pro
+++ b/src/proto/sound.pro
@@ -2,6 +2,6 @@
 void f_sound_playevent(typval_T *argvars, typval_T *rettv);
 void f_sound_playfile(typval_T *argvars, typval_T *rettv);
 void f_sound_stop(typval_T *argvars, typval_T *rettv);
-void f_sound_stopall(typval_T *argvars, typval_T *rettv);
+void f_sound_clear(typval_T *argvars, typval_T *rettv);
 void sound_free(void);
 /* vim: set ft=c : */