SidlWidl is really the only option. It is really buggy, but it allows you to move specific items around in specific windows and see the results without being in game. A lot of it doesn't work right and the project was abandoned 15+ years ago without ever being completed.
As Baler mentioned, Notepad++ and a hotkey to reload in game are what will benefit you the most though. I've never had a problem with crashing after reloading to much like a lot of people have, it may depend on the Titanium install you have or something like that.
In your Default UI folder there is a .doc file called "AboutSIDL" from the EQLive devs that gives a short run down on the XML that works with Titanium. The best way to learn is reverse engineering other UI's.