Move DropArea into Controllers namespace

In preparation for View/Controller split.

src/KDDockWidgets.h

@@ -39,7 +39,10 @@ namespace KDDockWidgets {
 DOCKS_EXPORT
 Q_NAMESPACE
 class MultiSplitter;
+
+namespace Controllers {
 class DropArea;
+}
 
 enum Location {
     Location_None,
@@ -173,7 +176,7 @@ private:
     friend class Layouting::Item;
     friend class Layouting::ItemBoxContainer;
     friend class KDDockWidgets::MultiSplitter;
-    friend class KDDockWidgets::DropArea;
+    friend class KDDockWidgets::Controllers::DropArea;
 
     InitialOption(DefaultSizeMode mode)
         : sizeMode(mode)