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/testdir/test_sound.vim b/src/testdir/test_sound.vim
index e47c677..1e2c455 100644
--- a/src/testdir/test_sound.vim
+++ b/src/testdir/test_sound.vim
@@ -41,7 +41,7 @@
   let id2 = sound_playfile(fname, 'PlayCallback')
   call assert_true(id2 > 0)
   sleep 20m
-  call sound_stopall()
+  call sound_clear()
   call assert_equal(id2, g:id)
   call assert_equal(1, g:result)
 endfunc