Have
```xml
<body t-name="webclient" />
```
and
```js
const comp = new WebClient();
comp.mount(document.body, {position: 'self'});
```
Before this commit, the body that was there before anything had happened
was *replaced* by the new body node created by the WebClient OWL component
After this commit, we ensure that the element body is the same at reference level
In most cases, we just want Component<any, Env>. But since it was so
annoying to have always the type Env, we actually used
Component<any,any> everywhere.
With this commit, the generic types have a default (and their order is
swapped), so we can simply use Component in most cases, and
Component<Props> when we want to type the props.
Géry Debongnie