I'm learning GTK+, it's nice but it is not as easy as I first though. It's a weird way to develop OO. So far I didn't like it but that might change in the future.
I decided to focus on the code itself and I could do a few things better in FLTK like changing the border type globally and decreasing font size.
Here it is how it's now:
I also added sort by column header in all lists, added some field validation, improved the workflow a little bit and fixed some bugs. For more info, please check the change log.
I really enjoy working on this little project and it help me keep my stuff in order. Please feel free to reach me or to fill bug reports or feature requests. Thank you.