Create gui.cpp

A simple GUI consisting of a file chooser(to select the program and song) and a way to generate a shell script to run the program via terminal.

To compile gui.cpp, type g++ gui.cpp -o gui `pkg-config gtkmm-3.0 --libs --cflags` on terminal in the working directory
To run, type ./gui on terminal in the working directory

Note: The shell script generated the first time need to be made executable. Type chmod 755 run.sh on terminal in the working directory

Dependencies: libgtkmm-3.0-dev (To install, type sudo apt-get install libgtkmm-3.0-dev on terminal)