Improve build and widget storage

There are no longer any qt headers included in wx/qt headers.
Applications do not need to link with qt librarys anymore, only wxqt libraries.
wxWindow and derived widgets only contain one pointer to their qtwidget, no longer
  carrying both base and derived pointers in parallel as was before.

include/wx/qt/taskbar.h

@@ -8,12 +8,13 @@
 #ifndef _WX_QT_TASKBAR_H_
 #define _WX_QT_TASKBAR_H_
 
-#include <QtWidgets/QSystemTrayIcon>
+class QSystemTrayIcon;
 
 class WXDLLIMPEXP_CORE wxTaskBarIcon : public wxTaskBarIconBase
 {
 public:
     wxTaskBarIcon(wxTaskBarIconType iconType = wxTBI_DEFAULT_TYPE);
+    virtual ~wxTaskBarIcon();
 
     // Accessors
     bool IsOk() const { return false; }
@@ -26,7 +27,7 @@ public:
     virtual bool PopupMenu(wxMenu *menu);
 
 private:
-    QSystemTrayIcon m_qtSystemTrayIcon;
+    QSystemTrayIcon *m_qtSystemTrayIcon;
 
     wxDECLARE_DYNAMIC_CLASS_NO_COPY(wxTaskBarIcon);
 };