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/gtk/webview_webkit.h

@@ -142,6 +142,7 @@ public:
     virtual void DeleteSelection();
     virtual bool HasSelection();
     virtual void SelectAll();
+    virtual wxString GetSelectedText();
 
     /** FIXME: hack to work around signals being received too early */
     bool m_ready;