[IMP] web: extend qweb to allow dynamic templates in t-call

Before this change the t-call can use only receive a literal as parameters:
<t t-call="template.name17" />
This call render the template 'template.name17' with the current context.

Now, we can defined an expression with braces (same syntax that t-attf):
<t t-call="template.name{{ 10 + 7 }}" />
This call render the template 'template.name17' with the current context.

This change allows for greater flexibility for applications using qweb.
(The performance due to this change is unchanged during rendering time.)