Setting window is still using Gtk3

The majority of GNOME 3+ distro ships with Gtk4 now.

Gtk4 provides many improvements over Gtk3. Most important of all, it provides dark theme support. GNOME can automatically switch between light and dark theme based on the hours. All Gtk4 windows' (if themed with Adwaita) header bar and theme color will be dark when applied and be coherent.

Meanwhile, the setup window of ibus-cangjie is still in Gtk3 and seems out of place