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.

interface/wx/webview.h

@@ -317,7 +317,7 @@ public:
     <a href="https://docs.microsoft.com/en-us/microsoft-edge/hosting/webview2">Edge WebView2</a>.
     It is available for Windows 7 and newer.
     The following features are currently unsupported with this backend:
-    virtual filesystems, custom urls, find.
+    virtual filesystems, custom urls.
 
     This backend is not enabled by default, to build it follow these steps:
     - Visual Studio 2015, or newer, is required