[FIX] component: avoid key collisions in sub components

This is a difficult part of owl: we need to be able to reconcile the
vdom generated (this is done with our virtual dom algorithm), but also
to be able to reconcile the proper components.

The issue here is that when we are in a list, with a t-key attribute on all
list nodes, and those list nodes contains sub component, then the
component system used the index of the list, and did not take into
account the key from the parent.  The fix is to keep track of the
current key in the context, and uses that as a part of the templateId.

src/qweb/qweb.ts

@@ -473,6 +473,7 @@ export class QWeb extends EventBus {
     if (node.nodeName !== "t") {
       let nodeID = this._compileGenericNode(node, ctx, withHandlers);
       ctx = ctx.withParent(nodeID);
+      ctx = ctx.subContext("currentKey", ctx.lastNodeKey);
       let nodeHooks = {};
       let addNodeHook = function(hook, handler) {
         nodeHooks[hook] = nodeHooks[hook] || [];
@@ -641,7 +642,9 @@ export class QWeb extends EventBus {
     let nodeID = ctx.generateID();
     let nodeKey: any = (<Element>node).getAttribute("t-key");
     if (nodeKey) {
-      nodeKey = ctx.formatExpression(nodeKey);
+      ctx.addLine(`const nodeKey${nodeID} = ${ctx.formatExpression(nodeKey)}`);
+      nodeKey = `nodeKey${nodeID}`;
+      ctx.lastNodeKey = nodeKey;
     } else {
       nodeKey = nodeID;
     }