Bug 1667674 - [Wayland] Support public primary-selection unstable protocol. r=stransky

This is required to support pasting the primary selection into Firefox on compositors only
supporting the public protocol, such as KWin. Getting the selection *from* Firefox is done
via GTK and will be supported from GTK 3.24.23 on.

The public protocol, while practically identical, will replace the gtk-private one eventually.
However, support for the private one will still be needed for a while.

Note: this also updates the auto-generated gtk-primary-selection files.

Differential Revision: https://phabricator.services.mozilla.com/D91594