nsIEventHandler in Firefox 3.5

 Handles an event. An nsIEventHandler can also get registered with with
 nsIPluginManager2::RegisterWindow and will be called whenever an event
 comes in for that window.

 Note that for Unix and Mac the nsPluginEvent structure is different
 from the old NPEvent structure -- it's no longer the native event
 record, but is instead a struct. This was done for future extensibility,
 and so that the Mac could receive the window argument too. For Windows
 and OS2, it's always been a struct, so there's no change for them.

 (Corresponds to NPP_HandleEvent.)

 @param aEvent   - the event to be handled
 @param aHandled - set to PR_TRUE if event was handled
 @result         - NS_OK if this operation was successful