We already use asciidoc for the gnome-extensions man page, while
the main man page is generated from docbook XML. The former is a
much friendlier source format, so use it for both man pages.
Hopefully the plain text format encourages updates, to prevent the
page from getting as badly out of sync again as it is currently.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1553>
Meson is on track to replace autotools as the build system of choice,
so support it in addition to autotools. If all goes well, we'll
eventually be able to drop the latter ...
https://bugzilla.gnome.org/show_bug.cgi?id=783229
We can autodetect what to do based on the user's session, so drop
the separate desktop files.
This, along with some future changes, will enable us to be bus
activatable.
https://bugzilla.gnome.org/show_bug.cgi?id=741666
