[REF] blockdom,compiler: implement properties

Before this commit, properties were just handled as a special case of
attributes. But it does not make that much sense, since they are
different. For example, the `readOnly` property does not have the same
name as the `readonly` attribute.  The confusion probably comes from the
fact that QWeb does not distinguish between property and attributes, so
Owl has to infer which one is which.

With this commit, we introduce a `block-property` directive in blockdom,
and change the compiler to use it in emitted code. It has the additional
benefits of slightly shrinking the runtime code, since the `isProp`
function can now be located in the compiler.

tests/blockdom/block_attributes.test.ts

@@ -145,4 +145,3 @@ test("class attribute (with a preexisting value", async () => {
   patch(tree, block([""]));
   expect(fixture.innerHTML).toBe(`<div class="tomato"></div>`);
 });
-