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().

src/popupwin.c

@@ -710,6 +710,15 @@ popup_create(typval_T *argvars, typval_T *rettv, create_type_T type)
     popup_mask_refresh = TRUE;
 }
 
+/*
+ * popup_clear()
+ */
+    void
+f_popup_clear(typval_T *argvars UNUSED, typval_T *rettv UNUSED)
+{
+    close_all_popups();
+}
+
 /*
  * popup_create({text}, {options})
  */
@@ -928,12 +937,6 @@ close_all_popups(void)
 	popup_close(curtab->tp_first_popupwin->w_id);
 }
 
-    void
-ex_popupclear(exarg_T *eap UNUSED)
-{
-    close_all_popups();
-}
-
 /*
  * popup_move({id}, {options})
  */