Make gwt-servlet.jar optionial

Currently the plugin always copies gwt-servlet.jar to your WEB-INF/lib directory.
But this jar file is only needed if you are working with servlets which we don't.

So it would be nice to have a checkbox somewhere to disable the need for gwt-servlet.jar

Actually the exact same comment can be made for the web.xml.
We don't use this file as we host our project in ISS which doesn't even understand that web.xml file.
If you just use GWT to communicate with a .NET, PHP, or something else backend over json or something like that the web.xml file is not needed.