community/slick-greeter: upgrade to 2.0.4
https://github.com/linuxmint/slick-greeter/commits/2.0.4
The patch to get rid of "gnome-common" can be drobbed, it's now implemented upstream.
New slick-greeter version 2.0.4 adds build support for meson. I didn't change the current autotools build. Should the build be changed to meson? Maybe in a separate MR after this one?