Make a proper use of Qt threads

Let's first read http://qt-project.org/doc/qt-5/thread-basics.html

It seems that the Qt framework is made for designing full multi-threaded applications, so we should leverage this.

At first we were mosty using Qt for the GUI part (QtGui module), so we need to refactor the app to use QThread (QtCore module) for all threads and make sure that the main (GUI) thread and the worker threads can communicate both ways making a proper use of Qt slots and signals.

Need to define a thread diagram to kown by which thread each thread is started: command line thread, GUI thread, DAO thread, ...