keybinder-3.0: Add package

Does the naming scheme (`keybinder_3_0`) seem reasonable?
I guess it should be ```name = "keybinder-3.0-${version}"``` (3.0 instead of 0.3)?

Hm... the keybinder / keybinder 3.0 package namings is confusing... will the 3.0 part ever be bumped to 3.1? Should we call it "keybinder3-${version}" instead, so that nix (and humans!) will not confuse the package names as much?