Graphical User Interface is a type of interface that has many graphics. Usually, it involves various metaphors, such as desktop or window. All modern consumer computers have GUIs, because it is thought people have little problems understanding them.

Can you recommend me a better GUI programming tool that fulfills my requirements? I know Qt has some sort of XML-like markup these days, but is it easier to develop in and lighter or faster than tkinter?

As the name suggests, Xdialog is a graphical clone of its text-based counterpart, that tries hard to be compatible (and the other way around). Sadly, its latest version is so old it uses Gtk 1.2! Good luck finding either packaged for a modern Linux distribution.

A more reliable option is Zenity, that's part of Gtk3 and usually preinstalled along with it. It has simplified command line options compared to (X)dialog, but in return features modern controls like a tabular list and desktop notifications. It also makes it much easier to use forms, for example. Plus, certain controls can receive their data on standard input and thus handle a lot more than if it came from the command line. ...

Last but not least, there's yad, a fork of Zenity with more controls, more field types and so on. Its last release was more than four years ago, but that's not a problem just yet.

Some cool screenshots here