Implement wxWindow::GetToolTipText().

This was declared in wx/window.h but somehow never implemented.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@61535 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/window.h

@@ -2216,6 +2216,11 @@ public:
     */
     wxToolTip* GetToolTip() const;
 
+    /**
+        Get the text of the associated tooltip or empty string if none.
+     */
+    wxString GetToolTipText() const;
+
     /**
         Attach a tooltip to the window.