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/popupwin.c b/src/popupwin.c
index 000bed9..c512369 100644
--- a/src/popupwin.c
+++ b/src/popupwin.c
@@ -711,6 +711,15 @@
 }
 
 /*
+ * popup_clear()
+ */
+    void
+f_popup_clear(typval_T *argvars UNUSED, typval_T *rettv UNUSED)
+{
+    close_all_popups();
+}
+
+/*
  * popup_create({text}, {options})
  */
     void
@@ -928,12 +937,6 @@
 	popup_close(curtab->tp_first_popupwin->w_id);
 }
 
-    void
-ex_popupclear(exarg_T *eap UNUSED)
-{
-    close_all_popups();
-}
-
 /*
  * popup_move({id}, {options})
  */