samples: Move 'ZMQContext' creation out of sample classes.
ZMQSocket,ZMQContext: Handle closing/deleting 'ZMQSocket' and 'ZMQContext' instances correctly.

Method 'ZMQSocket::close()' now is a slot and also unregisters instance from its 'ZMQContext' instance.
Deleting a 'ZMQContext' now doesn't delete 'ZMQSocket' instances anymore. Instead, the socket is closed and its pointer to the associated context is set to 0.
ZMQContext: Calling method 'createSocket' without specifying a parent pointer now doesn't implicitly set 'ZMQContext' instance as parent anymore.

CAUTION: This breaks compatibility with older code which assumes the socket instance is implicitly managed by its associated context instance if no parent is specified. So this causes memory leaks in such cases.
samples,refactoring: Use new 'SampleBase' class and simplify code.
samples: Use 'NZMQT_' prefix for include guards.
samples,PubSubServer: Use "normal" signal connection for timer.
samples,bugfix: Create sockets within their dedicated threads.
samples,PushPullSink: Remove compiler warning.
samples,PushPullSink: Code formatting.
bug fix,ZMQContext: Don't directly call a socket's 'close' method as such a call is only allowed from within the socket's thread.
samples: Simplify code and prepare it for running in thread.
testing: Added unit test project with a first (still failing!) test for PUP-SUB protocol.
testing,bug fix: Make unit test pass.
testing: Create 'ZMQContext' within unit test case.
testing: Simplify test code.
tests,pubsub: Fixed debug output.
samples: Prepare REQ-REP sample for unit testing.
testing,reqrep: Add unit test for REQ-REP protocol.
samples: Prepare PUSH-PULL sample for unit testing.
testing,pushpull: Add unit test for PUSH-PULL protocol.