[IMP] qweb: throw error if t-component not used on a node t

Before this rev, using t-component on a div (for example) node
would silently ignore the div and replace it by the root node of
the component. A way to improve this was to create an extra div
node and to put the component inside it. However, it raised
questions: what do we do with other attributes set on this tag?
Do we apply them on the div, or on the component? In addition,
some of them only make sense on the component (e.g. props), so we
have to detect them. Whatever we would have decided, it wouldn't
have been obvious from the user point of view, so we chose not to
support it, and we thus now raise an error in this case.

Closes #487.

doc/reference/component.md

@@ -1068,6 +1068,8 @@ class App extends Component<any, any, any> {
 In this example, the component `App` selects dynamically the concrete sub
 component class.
 
+Note that the `t-component` directive can only be used on `<t>` nodes.
+
 ### Asynchronous Rendering
 
 Working with asynchronous code always adds a lot of complexity to a system. Whenever