[FIX] qweb: fix issue variables set in body of t-call

The body of a t-call directive may be used to define private variables
to the sub template call.

However, the code that handles t-call worked like this:

- compile sub template if necessary
- then compile body of t-call to extract variables

This means that the variables defined in the t-call body were not yet
processed and available in the context.  Because of that, when the call
to t-esc is done, there is not internal qweb var, and the code simply
outputs a scope['varname'], which is in our case a VDOMArray, so it is
displayed as [object object]

What this fix does is changing the way t-esc works: if we are in the
context of a sub template, then it assumes that any outside variable may
or may not be a VDomArray, so it needs to check and eventually convert
it to a string, if necessary.

closes #719

src/qweb/qweb.ts

@@ -413,10 +413,12 @@ export class QWeb extends EventBus {
     name: string,
     elem: Element,
     parentContext?: CompilationContext,
-    defineKey?: boolean
+    defineKey?: boolean,
+    isSubTemplate?: boolean
   ): CompiledTemplate {
     const isDebug = elem.attributes.hasOwnProperty("t-debug");
     const ctx = new CompilationContext(name);
+    ctx.isSubTemplate = Boolean(isSubTemplate);
     if (elem.tagName !== "t") {
       ctx.shouldDefineResult = false;
     }