[FIX] component: subtle issue with unmounted children

Owl has to manage a lot of interesting situations.  One of them is when
a rendering is initiated, which creates a sub component, but then
another rendering starts, which invalidate the previous one, and will
create another sub component.  Since the first sub component was not
ever in the DOM, we cannot rely on the vdom patching process to remove
it, so we have to do it manually.

Sadly, this is actually a very tricky situation, since there are other
subtle situations where the code that remove an unmounted widget could
be executed, in particular when the parent component is unmounted, then
remounted, then modified to trigger yet another rendering.

In this commit, we handle this case more carefully by making sure that
the destroyed subcomponent properly configures its pvnode so the patch
process happens as expected.

joint work with the framework team, and in particular LPE for his work on
finding a testcase!

closes #724, #731

src/qweb/qweb.ts

@@ -216,8 +216,8 @@ export class QWeb extends EventBus {
   // id, and a (global) mapping from an id to the compiled function.  This is
   // necessary to ensure that global templates can be called with more than one
   // QWeb instance.
-  subTemplates: {[key: string]: number} = {};
-  static subTemplates: {[id: number]: Function} = {};
+  subTemplates: { [key: string]: number } = {};
+  static subTemplates: { [id: number]: Function } = {};
 
   isUpdating: boolean = false;
   translateFn?: QWebConfig["translateFn"];