Toy we know from childhood. Entirely written in C++ as side project.
You can download binaries from /bin folder, choosing your platform, OR compile project yourself from sources located in /src and /include.
You can create new project on your C++ IDE and add all files from /src and /include to it or compile from command line. Don't forget to link -ltgui -lsfml-graphics -lsfml-window -lsfml-system
Compilation won't suceed if you don't link TGUI and SFML libraries. To make sure everything goes right use -std=c++14 (or higher) compiler flag.
On linux you can download TGUI and SFML by typing
sudo apt-get install libsfml-dev
sudo add-apt-repository ppa:texus/tgui-0.8
sudo apt-get update
sudo apt-get install libtgui-dev
in your terminal.