[IMP] qweb: add support for event capture

In this commit, we uses the "!" as suffix to designate an event that
should be captured.  This is inspired by the Vue source code.

This solves the issue of communicating additional information to the
underlying virtual dom.  However, this will most likely disappear when
we rewrite the vdom as a virtual block system.

closes #650

doc/reference/event_handling.md

@@ -111,11 +111,12 @@ In order to remove the DOM event details from the event handlers (like calls to
 `event.preventDefault`) and let them focus on data logic, _modifiers_ can be
 specified as additional suffixes of the `t-on` directive.
 
-| Modifier   | Description                                                       |
-| ---------- | ----------------------------------------------------------------- |
-| `.stop`    | calls `event.stopPropagation()` before calling the method         |
-| `.prevent` | calls `event.preventDefault()` before calling the method          |
-| `.self`    | calls the method only if the `event.target` is the element itself |
+| Modifier   | Description                                                                                                              |
+| ---------- | ------------------------------------------------------------------------------------------------------------------------ |
+| `.stop`    | calls `event.stopPropagation()` before calling the method                                                                |
+| `.prevent` | calls `event.preventDefault()` before calling the method                                                                 |
+| `.self`    | calls the method only if the `event.target` is the element itself                                                        |
+| `.capture` | bind the event handler in [capture](https://developer.mozilla.org/en-US/docs/Web/API/EventTarget/addEventListener) mode. |
 
 ```xml
 <button t-on-click.stop="someMethod">Do something</button>