Add auto-declaring property constructor to WriteDecorHandleKey

The MultiThreadingEventdataAccess Twiki indicates that WriteDecorHandleKey should have a 4-argument constructor, which automatically declares the WriteDecorHandleKey as a propertyand allows for nice list-initialization (I'd like to use it in the jet reconstruction packages). But, it turns out this constructor doesn't actually exist, which seems to just be an oversight. This MR implements it. The template stuff confused me a bit, so I followed the example of WriteHandleKey.

Tagging @ssnyder and @leggett, who I believe are the authorities on this stuff.