ConnectionEvaluator: Allow reacting to newly queued invocations

[LeonMatthesKDAB]

Currently, there is no way for a framework like KDUtils to wake up the associated thread when something is queued on the ConnectionEvaluator.
This is problematic, as it may mean the thread may remain dormant, even if there is usueful work to be done.

So we want to give users of ConnectionEvaluator a way to react whenever something new was queued onto the evaluator.

Currently we have thought of two ways to do this:

1. Create a protected virtual function that is called after something is queued onto the ConnectionEvaluator
   • This would allow users to subclass the ConnectionEvaluator to react to changes
2. Add a signal to the ConnectionEvaluator that signals whenever something is queued

Subclassing seems to be the more flexible option, as it allows the ConnectionEvaluator to add condition variables or other data that may be required.

[LeonMatthesKDAB]

Complete as of #86