[FIX] qweb: t-call properly transfer key to sub components

Since t-call is now a function call, we need to properly handle the
internal key used to find previous components.  If a t-call is inside a
t-foreach, then we need to transfer the key to the sub template.
Otherwise, each component in the subtemplate will be associated to the
same key, which means that it will lead to big issues: components are
destroyed and reused...

closes #581

src/qweb/qweb.ts

@@ -383,7 +383,12 @@ export class QWeb extends EventBus {
     });
   }
 
-  _compile(name: string, elem: Element, parentContext?: CompilationContext): CompiledTemplate {
+  _compile(
+    name: string,
+    elem: Element,
+    parentContext?: CompilationContext,
+    defineKey?: boolean
+  ): CompiledTemplate {
     const isDebug = elem.attributes.hasOwnProperty("t-debug");
     const ctx = new CompilationContext(name);
     if (elem.tagName !== "t") {
@@ -396,6 +401,10 @@ export class QWeb extends EventBus {
       ctx.hasParentWidget = true;
       ctx.shouldDefineResult = false;
       ctx.addLine(`let c${ctx.parentNode} = extra.parentNode;`);
+      if (defineKey) {
+        ctx.currentKey = `key${ctx.generateID()}`;
+        ctx.addLine(`let ${ctx.currentKey} = extra.key || '';`);
+      }
     }
     this._compileNode(elem, ctx);