GTK4 Transition Part 2: Sugar Shell

The Sugar Shell is the core desktop environment of Sugar and is responsible for activity launching, the Journal, the Frame, and system integration through DBus. The current Sugar Shell is built on GTK3, and several core components still depend on deprecated GTK3 APIs, which must be migrated to GTK4. This project focuses on the GTK4 migration of the Sugar Shell. The work includes replacing deprecated GTK3 APIs, migrating container and layout APIs to GTK4 equivalents, updating event handling using GTK4 event controllers, migrating styling to GTK CSS, and updating display and geometry handling using modern GTK4 display APIs. The migration will be done incrementally, component by component, starting with core components such as the Frame, Home View, Activity Launcher, Journal, Clipboard, and Control Panel. Each component will be migrated, tested in a running Sugar session, and submitted as small patches to ensure that the Sugar Shell remains stable and usable throughout the migration process. By the end of the project, the Sugar Shell will run on GTK4, core components will be migrated to GTK4 APIs, deprecated GTK3 APIs will be removed from migrated components, and migration documentation will be provided to support the remaining GTK4 transition work in the Sugar project.