Widget toolkits are libraries used to create the graphical user interface (GUI) of programs.
GTK+ - 1998
wxWidgets - 1992
Qt - 1991
Tk - 1991
Reference
Creating a Very Simple GUI System for Small Games - 2014
Why I think Immediate Mode GUI is way to go for GameDev tools - 2017
Sol on Immediate Mode GUIs (IMGUI) - 2006