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/osx/webview_webkit.h

@@ -56,6 +56,7 @@ public:
     virtual void Reload(wxWebViewReloadFlags flags = wxWEB_VIEW_RELOAD_DEFAULT);
     virtual void Stop();
     virtual wxString GetPageSource();
+    virtual wxString GetPageText() { return ""; }
     virtual void SetPageTitle(const wxString& title) { m_pageTitle = title; }
     virtual wxString GetPageTitle(){ return m_pageTitle; }