Drop "private" subfolders

"private" subfolders violate the structure of the project,
since the existence of all other subfolders is based on a
different principle.
In addition, there is no clear line between "private" and
"non private".

src/gui/tristateaction.cpp

@@ -31,7 +31,7 @@
 #include <QString>
 #include <QWidget>
 
-#include "private/tristatewidget.h"
+#include "tristatewidget.h"
 
 TriStateAction::TriStateAction(const QString &text, QWidget *parent)
     : QWidgetAction {parent}