Unable to use add-ons: Widgetset does not contain implementation

I’m facing a critical problem in my Vaadin project (which has otherwise worked fine so far).
I’m unable to use any add-on, I’m following the very simple instructions for the Eclipse plugin, but when running the application always get the dreaded error:

The add-on in case is the
one if it matters, but the problem happens with any other add-on.

There seems to be something not quite right with the Eclipse plugin, or the project. After dragging the add-on jar to Eclipse, it will offer to recompile the widgetset as expected but will also ask:

is that normal?

Also, the message

never seems to go away.

The xml file generated for the widgetset looks like this:

   <inherits name="com.vaadin.terminal.gwt.DefaultWidgetSet" />
    <inherits name="org.batista.MyprojectWidgetset" />

    <inherits name="org.vaadin.googleanalytics.tracking.GoogleAnalyticsWidgetset" />

The page at
Using Vaadin Add-ons
does not offer much guidance…

Any help to solve or debug this is much appreciated.

Ok, after taking a look at the instructions for Maven
(although I’m not using Maven), I manually added the following to web.xml, and things appear to be working now: