We started to design such a tool in 1998. Now it's on the market since 1999 and we made a lot of good experiences by using it in our projects.
Look at
http://www.jeasy.de and download the evaluation version.
Open the Repository and run the Example. All GUI elements are stored in XML. The Repository let you define a lot of properties for most SWING objects.
A preview function let you see at once you have designed.
A HTTPConnector is used for C/S computing over Internet.
The automation API let you write
testing scripts and run them on your application.
And so on ....
I hope you will find what you are looking for.