On wayland applications are not trusted to perform screen grabs by
default, it is however possible to let the user specifically pick the
color of a pixel using the XDG desktop portal (otherwise used for
sandboxing etc.). Try to use this portal on unix systems by default.
To support this use case some extra abstraction is necessary as this
constitutes a platformservice rather than a platform feature. To that
end the QPlatformService has gained a capability system and a pure
virtual helper class to facilitate asynchronous color picking. When
supported the color picking capability takes precedence over the custom
picking code in QColorDialog.
Fixes: QTBUG-81538
Change-Id: I4acb3af11d459e9d5ebefe5abbb41e50e3ccf7f0
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>