This is essentially a commandline implementation of the client-side of
the org.freedesktop.Application D-Bus interface.
It includes support for tab-completion based on desktop files and their
contents.
https://bugzilla.gnome.org/show_bug.cgi?id=704218
The bash-completion code nowadays expects completion files to
be installed in /usr/share/bash-completion/completions, and
expects them to be named like the command they are completing
for.
https://bugzilla.gnome.org/show_bug.cgi?id=677782