Add support for retrieving the currently selected text. Implement on all backends, document and unit test.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/SOC2011_WEBVIEW@68220 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/osx/webview_webkit.h

@@ -106,10 +106,9 @@ public:
     virtual void DeleteSelection();
     virtual bool HasSelection() { return false };
     virtual void SelectAll() {};
+    virtual wxString GetSelectedText();
 
     // ---- methods not from the parent (common) interface
-    wxString GetSelectedText();
-
     wxString RunScript(const wxString& javascript);
 
     bool  CanGetPageSource();