Add GetPageText to MSW IE and to GTK WebKit. Add stub for OSX and document.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/SOC2011_WEBVIEW@68250 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/gtk/webview_webkit.h

@@ -110,6 +110,7 @@ public:
     virtual wxString GetCurrentURL();
     virtual wxString GetCurrentTitle();
     virtual wxString GetPageSource();
+    virtual wxString GetPageText();
     //We do not want to hide the other overloads
     using wxWebView::SetPage;
     virtual void SetPage(const wxString& html, const wxString& baseUrl);