I'm building the options panel for my addon.
If I want add controls that are not available with xml as the trackbar (just like this) what can I do?
I know that must use an external library, but wich is it?
You mean the Slider widget? The Blizzard UI uses XML for all of their windows/options. I'm fairly positive you can do it.
You don't need to use a library to make config windows. And you don't have to use XML (you could write it all in Lua).
That said, I create my larger, more extensive option windows use AceConfig. If the addon only has a few settings, though, I write it from scratch.
"You'd be surprised how many people violate this simple principle every day of their lives and try to fit square pegs into round holes, ignoring the clear reality that Things Are As They Are." -Benjamin Hoff, The Tao of Pooh