Add filter criteria parameter to URL used to access the custom widgets page

Currently, filter criteria set in the custom widget page are not captured as an url parameter. Hence, when exiting from a widget or going back via the browser, the filter criteria are lost and one has to re-enter the filter value manually.

As the list of custom widgets grows, this is becoming very cumbersome.

It would be nice if the filter criteria is kept, i.e. by reading it directly from the URL.