Issue #1792 - Part 3: Add ConstructibleEventTarget helper class.

Of course it couldn't be trivial :P
wrapObject is a pure virtual function which prevents an abstract constructor
from being used.
So, we need to extend DOMEventTargetHelper (applicable acronym: DETH :D) with
a wrapObject implementation - in a derivative class is the clearest way to do
this, as opposed to making DETH more complex.

dom/events/moz.build

@@ -40,6 +40,7 @@ EXPORTS.mozilla.dom += [
     'ClipboardEvent.h',
     'CommandEvent.h',
     'CompositionEvent.h',
+    'ConstructibleEventTarget.h',
     'CustomEvent.h',
     'DataContainerEvent.h',
     'DataTransfer.h',
@@ -80,6 +81,7 @@ SOURCES += [
     'ClipboardEvent.cpp',
     'CommandEvent.cpp',
     'CompositionEvent.cpp',
+    'ConstructibleEventTarget.cpp',
     'ContentEventHandler.cpp',
     'CustomEvent.cpp',
     'DataContainerEvent.cpp',