qt: created standalone article with configuration guide

wiki/linux/qt.md

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+# Qt
+
+[Qt](https://www.qt.io/) is an application and widget toolkit.
+
+## Setup
+
+On most linux distributions different Qt versions are installed as a dependency of various programs
+that use graphical user interfaces.
+
+### Configuration
+
+The configuration can easily be achieved by using the programs
+[qt5ct](https://github.com/desktop-app/qt5ct) or [qt6ct](https://github.com/trialuser02/qt6ct)
+the latter working with the current version of Qt.
+qt5ct is officially archived and qt6ct is the recommended program to use and will mainly be
+discussed in this article.
+These programs can sometimes be found as [package](/wiki/linux/package_manager.md) of the same
+name.
+Afterwards `qt6ct` can be run to get a graphical user interface to set the theme and icons of Qt
+applications.
+This will create or edit a configuration file located at `~/.config/qt6ct/qt6ct.conf`.
+To make this config apply for all Qt applications the following line needs to be inserted into the
+`~/.profile` file.
+
+```sh
+export QT_QPA_PLATFORMTHEME="qt6ct"
+```