Migrate Pitivi from GTK3 to GTK4

My proposal aims toward migrating Pitivi from GTK3 to GTK4. Pitivi’s UI is in GTK, an open-source, cross-platform widget toolkit. Currently, Pitivi uses GTK 3.0 but the current new and stable version of GTK is GTK 4.0, at some point in time GTK3.0 will be deprecated and hence it is necessary to migrate Pitivi to GTK 4.0. The ultimate goal of my project will be to continue migrating Pitivi’s codebase to GTK 4.0 runtime and then migrate the app to GTK 4.0. It also includes fixing any critical and non-critical bugs that appear during migration to ensure the smooth functioning of Pitivi.