Core without qwidget

Created by: nniclausse

Currently, dtkCore depends on QWidgets, hence links to openGL, this can be troublesome if a dtk based app want's to link on mesa (paraview plugin). This PR removes the dependencie of dtkCore and dtkDistributed on QWidget, and adds an option to disable dtkWidgets in cmake. For this, we move dtkApplication class to dtkWidgets, reintroduce dtkCoreApplication class, base dtkDistributedApplication class on dtkCoreApplication, and create a new dtkDistributedGuiApplication in dtkWidgets. We factorize the code into two new classes dtkApplicationPrivate and dtkDistributedApplicationPrivate (is it a good naming ?), to avoid the previous ugly (and partially removed) design using multiple inheritance