Freedesktop (XDG) portals are a collection of D-Bus APIs that work
across desktop environments, display servers and work within
containerized applications, like Flatpak. The internal implementation
can then choose to implement these in such a way that takes into account
security considerations, as well as making sure the user consents to
certain actions.
One such portal is the `Screenshot` portal, which contains a
`Screenshot()` method as well as `PickColor()`. We already use the
former for taking a screenshot, and this commit makes sure our color
picker also makes use of the latter.
By doing this, color picking is now possible on the major Wayland
compositors.
(Honestly, we should remove DE-specific backends like that of KWin, to
have less variation on the possible results of a color picking
operation).
Fixes https://gitlab.gnome.org/GNOME/gimp/-/issues/1074
Niels De Graef