Port Swatches Panel to use GtkSnapshot Rendering
Refactored ColorItem to inherit from Gtk::Widget and implemented the snapshot_vfunc for GTK4 compatibility.
Replaced the previous Cairo drawing calls with Gtk::Snapshot methods (append_color and similar).
Removed the set_draw_func method from common_setup in ColorItem, transitioning it to the GtkSnapshot rendering model.
This refactoring helps prepare the Swatches panel for GTK4’s new rendering model.