Add InternalFlag_DontShowWhenUnfloatingHiddenWindow

It's an internal knob you can turn so that setFloating(false)
doesn't show the dockwidget. Only adding this flag in case you need
to support both KDDW and QDW at the same time, or during porting/evaluation.

Fixes #125

src/DockWidgetBase.cpp

@@ -276,6 +276,12 @@ bool DockWidgetBase::setFloating(bool floats)
     if ((floats && alreadyFloating) || (!floats && !alreadyFloating))
         return true; // Nothing to do
 
+    if (!floats && (Config::self().internalFlags() & Config::InternalFlag_DontShowWhenUnfloatingHiddenWindow) && !isVisible()) {
+        // Mimics behaviour of QDockWidget, which you might need during porting.
+        // Not something we suggest though. For KDDW, setFloating(false) means dock, and that implies showing.
+        return false;
+    }
+
     if (floats) {
         d->saveTabIndex();
         if (isTabbed()) {