gnome-settings-daemon has been split up into separate daemons, which
means we'll need to invoke those separately.

See https://bugzilla.gnome.org/show_bug.cgi?id=772370

https://bugzilla.gnome.org/show_bug.cgi?id=772408

gnome-shell and gnome-shell-wayland have been consolidated, and
now get dbus activated.  The new name is org.gnome.Shell.

This commit changes the session files to reference org.gnome.Shell.

https://bugzilla.gnome.org/show_bug.cgi?id=741666

We now have no fallback mode, so there's no point in running
the IsRunnableHelper.

https://bugzilla.gnome.org/show_bug.cgi?id=672943

We no longer support fallback mode in 3.8, so we don't need this
session definition anymore. We are keeping the runnable helper,
since it sets some X properties that we need.

https://bugzilla.gnome.org/show_bug.cgi?id=687586

For gnome-shell and gnome-panel, we really know this is what we want. We
don't want another provider.

All components defined in .session files are really required, and the
format should make this clear.

Rename:
 - Required to RequiredProviders
 - Required-* to DefaultProvider-*
 - DefaultApps to RequiredComponents

Make sure to check for all required components and providers at all
time, and reduce code redundancy a bit.

It would make sense to run gnome-session-is-accelerated in
GDM, in parallel with the user is typing their password.  A
good way to pass that knowledge through to gnome-session
so it knows it can avoid repeating the same check is via
an X property.

So create a new tool "gnome-session-check-accelerated", which takes
the name of the old one (now renamed "gnome-session-check-accelerated-helper")
and stores that status as an X property.  gnome-session will then
itself run it again, but it should pick up the property and reuse that
value.

https://bugzilla.gnome.org/show_bug.cgi?id=641992

Since this is what it is...

Since nautilus is no longer showing icons on the desktop by default,
running it only slows down login.

We use the gnome-session-is-accelerated tool to know if GNOME Shell be
used, and if no, we fallback to the classic-gnome session.

This will help us both for the GSettings migration and to implement the
fallback from GNOME Shell to Classic GNOME.

The --default-session-key argument has been changed to --session, which
takes the basename (without extension) of the .desktop-like file to use.
By default, gnome is used, and the gnome.session file is looked for in
$XDG_CONFIG_HOME/gnome-session/sessions,
$XDG_CONFIG_DIRS/gnome-session/sessions,
$XDG_DATA_DIRS/gnome-session/sessions.