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/window.h

@@ -173,6 +173,11 @@ public:
 
     virtual QScrollArea *QtGetScrollBarsContainer() const;
 
+#if wxUSE_TOOLTIPS
+    // applies tooltip to the widget.
+    virtual void QtApplyToolTip(const wxString& text);
+#endif // wxUSE_TOOLTIPS
+
 protected:
     virtual void DoGetTextExtent(const wxString& string,
                                  int *x, int *y,