Draft - Multi-Threaded bindings

[LeonMatthesKDAB]

Use Thread 1 with normal signal evaluation to mark a binding as dirty.

Then have all the dependent properties associated with a BindingEvaluator.
This binding evaluator would then be evaluated by Thread 2 (typically GUI Thread), so the evaluation actually happens on a separate thread.

For this to work the following would need to be thread-safe:

1. Marking of a binding as dirty.
2. Reading from a Property & updating the bindings internal cache
3. Disconnecting/Connecting of Signals when creating a Binding

[LeonMatthesKDAB]

As we now support #23 , maybe that helps this problem as well.