Add a slot to mainwindow but amend to use Q_SLOTS

For some reason, 'private slots:' causes a build error but private
Q_SLOTS: works.

The error was that 'slots' did not name a type and it appeared to be
insensitive to whether the Makefile rule for .moc was in its current
place or preceeded the rule for .cpp.

Add a slot using the connectByName idiom e.g. actionNew connects to slot
on_actionNew_triggered(). Use qDebug to show this fires if the menu
option is selected.

Above to demonstrate how to begin to link menu to code paths.

Signed-off-by: Amit Chaudhuri <amit.k.chaudhuri@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>