perf-tool: Allow to run as a Wayland compositor

On Wayland, the display server is the Wayland compositor, i.e. gnome-shell itself.

As a result, we cannot spawn gnome-shell-perf-helper before gnome-shell is started, as gnome-shell-perf-helper needs to connect to the display server.

So, instead of spawning gnome-shell-perf-helper from the Python script gnome-shell-perf-tool, start it from gnome-shell itself.

Also, using the --replace option which does not work when gnome-shell is a Wayland compositor.

Add a --wayland option to gnome-shell-perf-tool so that it can spawn gnome-shell as a Wayland compositor so the whole perf tool can be starred from a console with:

  $ dbus-run-session -- gnome-shell-perf-tool --wayland

For testing purpose, --nested is also added, but the performance reported when gnome-shell runs as a nested Wayland compositor might not be significant:

  $ dbus-run-session -- gnome-shell-perf-tool --nested