[FIX] qweb: no more duplicated nodes during transitions

If a node was re-added while being removed (i.e. during the remove
transition), the node was duplicated in the DOM for the delay of
the remove transition. This rev. removes the old occurence directly
in that case.

Fixes #121

tools/playground/samples.js

@@ -187,7 +187,7 @@ const ANIMATION_CSS = `button {
 
 .flash {
     background-position: center;
-    transition: background 0.5s;
+    transition: background .6s;
 }
 
 .flash:active {
@@ -208,7 +208,7 @@ const ANIMATION_CSS = `button {
 }
 
 .fade-enter-active, .fade-leave-active {
-    transition: opacity .5s;
+    transition: opacity .6s;
 }
 .fade-enter, .fade-leave-to {
     opacity: 0;