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Source code
JQL GTK is an open source project hosted by SourceForge.net, so you can download its source code and do all you want with it, in the terms of GNU GPL license.
You can download the C/C++ sources of JQL GTK (and all source files, like images and resources) at project downloads page on SourceForge.net.
Sources files are organized in a Dev-C++ project, but you can modify and compile them with all standard IDEs and compilers (the original releases are compiled with GCC, the GNU C++ Compiler).