Add ClearSelection for msw ie and gtk webkit, with a stub for osx webkit. Document and add to unit tests.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/SOC2011_WEBVIEW@68274 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/osx/webview_webkit.h

@@ -109,6 +109,7 @@ public:
     virtual void SelectAll() {};
     virtual wxString GetSelectedText();
     virtual wxString GetSelectedSource() { return ""; }
+    virtual void ClearSelection() {}
 
     // ---- methods not from the parent (common) interface
     wxString RunScript(const wxString& javascript);