Fix changing tooltip via wxToolTip::SetTip() in wxQt

Update the actual tool tip used by Qt and not just the internal variable
when SetTip() is called.

Also implement wxToolTip::SetWindow().

Closes https://github.com/wxWidgets/wxWidgets/pull/1175

include/wx/qt/tooltip.h

@@ -10,7 +10,8 @@
 
 #include "wx/object.h"
 
-class wxWindow;
+class WXDLLIMPEXP_FWD_CORE wxWindow;
+
 class WXDLLIMPEXP_CORE wxToolTip : public wxObject
 {
 public: