Dev issue 6

Hello, I implemented what was explained in issue #6 (closed).

I tried to follow coding style.

I have tested the changes on my PC:

• Linux Mint 19.3 Tricia
• Linux fred-laptop 4.15.0-88-generic #88-Ubuntu SMP Tue Feb 11 20:11:34 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux

Also note that I used vala compiler 0.40.17 without any issue to generate the executable.

There were only few warnings regarding deprecated GTK calls, const mismatch, etc.

I may manage to solve them if needed, just ask!