Implement MouseEvent buttons attribute.

Source/platform/PlatformEvent.h

Index: Source/platform/PlatformEvent.h
diff --git a/Source/platform/PlatformEvent.h b/Source/platform/PlatformEvent.h
index a3dd410df194c82cc82459f922f6d0991d0405d0..563d7d77a7514c7ec7cfdfa23f9f915434c74eda 100644
--- a/Source/platform/PlatformEvent.h
+++ b/Source/platform/PlatformEvent.h
@@ -78,6 +78,10 @@ public:
         CtrlKey     = 1 << 1,
         MetaKey     = 1 << 2,
         ShiftKey    = 1 << 3,
+
+        LeftButtonDown   = 1 << 6,
+        MiddleButtonDown = 1 << 7,
+        RightButtonDown  = 1 << 8,

[Rick Byers, 2014/11/28 17:36:02]

By the way, with the addition of these, I believe it should now be possible to
remove the (redundant and confusing) PlatformMouseEvent::m_modifierFlags.  Can
you please add a FIXME there saying it's redundant and should be removed?  Let
me know if you're interested in that CL too - otherwise I can do it (I _think_
it should be easy, but I haven't traced all the code to verify).

[zino, 2014/12/03 15:47:15]

Done.

     };
 
     Type type() const { return static_cast<Type>(m_type); }