GSSDP 1.4.0

A new stable release of GSSDP.

Notable dependency changes compared to GSSDP 1.2:
- The sniffer application has been ported to GTK4
- The documentation is now generated using gi-docgen
- Minimum GLib version is bumped to 2.54

1.4.0
=====

- Fix building the sniffer when built as subproject
- Do not build docs by default

All contributors to this release:
- Jens Georg