Expose more private API so users can use the full power

Installed more private headers.
TabWidget_p.h was moved into widgets/ as the QtQuick impl won't
use that abstraction. Could even be folded into TabWidgetWidget.

Fixes #65

src/private/widgets/TabWidgetWidget_p.h

@@ -19,7 +19,8 @@
 #ifndef KDTABWIDGETWIDGET_P_H
 #define KDTABWIDGETWIDGET_P_H
 
-#include "../TabWidget_p.h"
+#include "TabWidget_p.h"
+
 #include <QTabWidget>
 
 namespace KDDockWidgets {