Define event constructing steps

This hook can be used by complex Event subclasses, such as KeyboardEvent and TouchEvent, to initialize their internal state upon creation.

Follow-up: w3c/uievents#194 and w3c/touch-events#94.

Fixes #414.

dom.bs

@@ -796,6 +796,14 @@ method must, when invoked, run these steps:
 
 <h3 id=constructing-events>Constructing events</h3>
 
+<a lt="Other applicable specifications">Specifications</a> may define
+<dfn export id=concept-event-constructor-ext>event constructing steps</dfn> for all or some
+<a for=/>events</a>. The algorithm is passed an <var>event</var> as indicated in the
+<a>inner event creation steps</a>.
+
+<p class=note>This construct can be used by {{Event}} subclasses that have a more complex structure
+than a simple 1:1 mapping between their initializing dictionary members and IDL attributes.
+
 <p>When a <dfn export for=Event id=concept-event-constructor>constructor</dfn> of the {{Event}}
 interface, or of an interface that inherits from the {{Event}} interface, is invoked, these steps
 must be run, given the arguments <var>type</var> and <var>eventInitDict</var>:
@@ -870,6 +878,8 @@ correct defaults.</p>
  <var>event</var> has an attribute whose <a spec=webidl>identifier</a> is <var>member</var>, then
  initialize that attribute to <var>value</var>.
 
+ <li><p>Run the <a>event constructing steps</a> with <var>event</var>.
+
  <li><p>Return <var>event</var>.
 </ol>