[IMP] component: only useState on props that are already reactive

Previously, components would automatically call useState on their props,
so that changes deeply within props would automatically cause the
component to be rendered. This can be useful when passing a piece of
state to children or descendants.

One problem with this is that all props implicitly become reactive, even
if the object that was passed as a props was not. The problem with that
being that since the original object is not reactive, any change made by
the parent will not go through the reactivity system and the children
won't be notified of the change, in essence, this reactive object is
essentially useless, while having a real cost: traversing reactive
objects creates more reactive objects, and those objects are all
proxies. This is expensive for basically no benefit, while also making
it more difficult to debug code that involves those objects.

This commit fixes that by only calling useState on objects that are
already reactive, allowing the usecase described in the first paragraph
without the drawbacks described in the second.

doc/reference/reactivity.md

@@ -26,9 +26,9 @@ To solve this issue, Owl provides two reactivity primitives:
 Most of the time, the `useState` hook is the best solution.
 
 Since version 2.0, Owl applies the fine grained reactivity at the component
-level: props are automatically turned into reactive object, so Owl can track
-which part of these props are consumed by each component, and is therefore able
-to only rerender the impacted components.
+level: reactive objects received as props are automatically subscribed to by the
+component, so Owl can track which part of these props are consumed by each
+component, and is therefore able to only rerender the impacted components.
 
 ## `useState`