It looks like the SynchronizationContext can now change on the same thread
(see http://msdn.microsoft.com/en-us/library/system.windows.basecompatibilitypreferences.reusedispatchersynchronizationcontextinstance%28v=vs.110%29.aspx)
meaning that we can no longer do a reference comparison between SynchronizationContext.Current
and the captured SynchronizationConext to check whether a dispatch is required.
It turns out we shouldn't have been doing this anyway.... So switch to using a Dispatcher,
which does support this stuff.
Additionally, Execute now uses an IDispatcher, which means the implementation can be switched
again in the future. It also makes unit testing easier....
Antony Male