Gitg moving to GTK4

GNOME Gitg is a graphical user interface for Git that aims to be a small, fast, and easy-to-use tool for visualizing repository history and managing projects. It is currently built on the GTK 3 toolkit, which is expected to be deprecated in the future. This proposal focuses on migrating Gitg to GTK 4, ensuring long-term maintainability, better performance, and a more modern user experience.