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

interface/wx/webview.h

@@ -226,6 +226,11 @@ public:
     */
     virtual wxString GetPageSource() = 0;
     
+    /**
+        Get the text of the current page.
+    */
+    virtual wxString GetPageText() = 0;
+    
     /**
         Returns whether the web control is currently busy (e.g. loading a page).
     */