Fix ColorPickerButton to work on Wayland
Currently ColorPickerButton
works by getting a screenshot of the entire screen to be able to pick a color, but this is not allowed on Wayland.
We should use the new Gtk.ColorPicker when it will be available: https://github.com/GNOME/gtk/blob/master/gtk/gtkcolorpicker.c
For a similar issue, see World/gcolor3#38 (closed)