GSSDP 1.4.0

[Jens Georg <mail jensge org>]

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

All contributors to this release:
 - Jens Georg <mail jensge org>