keybinder-3.0: Add package

keybinder-3.0 is for GTK+ 3, whereas keybinder (already in nixpkgs) is
for GTK+ 2.

This adds version 0.3.0 of keybinder-3.0, which we named "keybinder3" in
nixpkgs to (hopefully) lessen the confusion about what is package name
and what is version number.

[Bjørn: extend commit message beyond summary line]