Provide generic wxWebView::Find() implementation

This implementation is based on javascript 'window.find()'
https://developer.mozilla.org/en-US/docs/Web/API/Window/find
It's not standard, but implemented in most browsers.

It's not a complete implementation as it can't determine how
many search results can be found.

If the backends API provides a native find interface it should be
used instead of this implementation, but it's good fallback.

include/wx/webview.h

@@ -257,6 +257,7 @@ private:
     static wxStringWebViewFactoryMap::iterator FindFactory(const wxString &backend);
 
     bool m_showMenu;
+    wxString m_findText;
     static wxStringWebViewFactoryMap m_factoryMap;
 
     wxDECLARE_ABSTRACT_CLASS(wxWebView);