This commit introduces major improvements to QWeb: parametric `t-call`,
lazy XML evaluation.

### 1. Parametric `t-call`
Previously, parameters passed to a template through `t-call` were defined
inside the `t-call` tag using `t-set` elements. The content inside the `t-call`
was evaluated within the context of the called template.

Now, `t-call` becomes **parametric**:
- Parameters are passed directly as **attributes** on the `t-call` tag instead
  of using internal `t-set` definitions.
- The content inside the `t-call` tag no longer has access to these parameters;
  it only sees the variables available **before the `t-call`**.
- The attributes passed to `t-call` are used **only for rendering the called
  template** and **not** for the content inside the `t-call`.

**Example before:**
```xml
<t t-call="my_template">
    <t t-set="custom" t-value="x + y" />
    <t t-set="add_class" t-valuef="o_toto_{{stuff}}" />
    <t t-set="title">Hello <t t-out="user.name"/></t>
</t>
```
**Example after:**
```xml
<t t-call="my_template"
    custom="x + y"
    add_class.f="o_toto_{{stuff}}"
    title.translate="Hello {{user.name}}" />
```

### 2. Lazy XML evaluation & streamable rendering
QWeb now supports **lazy XML evaluation**:
- All XML content is now **lazy** and only evaluated **when explicitly
  rendered**, typically when using `t-out`.
- This also applies to XML content declared **inside `t-call` blocks**: the
  content is generated **only when actually output**.
- The variables available for evaluation are those present **at the time the
  content was declared**, not when it is rendered.
- As a result, **QWeb templates are now streamable**, allowing more efficient
  and predictable rendering.

### Impact
- Simplifies and clarifies `t-call` usage.
- Prevents unexpected side effects on local contexts.
- Enables **lazy evaluation** and **streamable QWeb rendering**.
- Support translatable formated string on `t-call` parameters.
- Improves predictability, and maintainability of QWeb templates.

### NB
`t-call` on other element than <t> is now forbidden.
The parametric t-call change is backward compatible (the stream is apply for t-call with attributes/params or t-call that does not contains t-set), the old semantics will be deprecated in 19.1 (with a log warning).
The new versions are the ones with attributes (not prefixed with t-*) on the t-call tag.