Clean up wxDropTarget and wxWindow a bit more by moving DnD stuff fully into drop target (besides connecting and disconnecting).

Window doesn't have to know more than how to hook itself up.
Impl now performs function of event filter/adapter.

include/wx/qt/dnd.h

@@ -10,8 +10,6 @@
 
 #define wxDROP_ICON(name)   wxICON(name)
 
-class QMimeData;
-
 class WXDLLIMPEXP_CORE wxDropTarget : public wxDropTargetBase
 {
 public:
@@ -24,10 +22,8 @@ public:
 
     wxDataFormat GetMatchingPair();
 
-    void OnQtEnter(QEvent* event);
-    void OnQtLeave(QEvent* event);
-    void OnQtMove(QEvent* event);
-    void OnQtDrop(QEvent* event);
+    void ConnectToQWidget(QWidget* widget);
+    void DisconnectFromQWidget(QWidget* widget);
 
 private:
     class Impl;