[GTK] Fix response_received events when there's no request_sent event (#1784)

For the moment, I use the following workaround:

    window.events.request_sent += bool

Author: jmuchemb

webview/platforms/gtk.py

@@ -365,12 +365,12 @@ def on_response(self, resource, _):
         self.pywebview_window.events.response_received.set(response_)
 
     def on_request(self, webview, resource, request):
-        if len(self.pywebview_window.events.request_sent) == 0:
-            return
-
         if len(self.pywebview_window.events.response_received) > 0:
             resource.connect('notify::response', self.on_response)
 
+        if len(self.pywebview_window.events.request_sent) == 0:
+            return
+
         headers = request.get_http_headers()
         original_headers = self._headers_to_dict(headers)
         url = request.get_uri()