Setup Makefile for Qt/C++ builds

Setup the build variables for building with QtWidgets, and add rules for
processing the Q_OBJECT macros with moc and generate widget code from
.ui files with uic.

Signed-off-by: Alberto Mardegan <mardy@users.sourceforge.net>
Have some C++ file in the project

Rename gtk-gui.c to qt-gui.cpp, and make the necessary changes so that
the project still builds.

Signed-off-by: Alberto Mardegan <mardy@users.sourceforge.net>
Introduce QApplication

Instantiate a QApplication and let Qt handle the event loop.
Add a QTranslator subclass to translate the UI via gettext.

Signed-off-by: Alberto Mardegan <mardy@users.sourceforge.net>