perf-tool: Allow to run as a Wayland compositor

perf-tool: Allow to run as a Wayland compositor

`gnome-shell-perf-tool` is initially designed to run on X11, using the
`--replace` option which does not work when gnome-shell is a Wayland
compositor.

A solution would be to run `gnome-shell-perf-tool` in place of just
`gnome-shell` to run the entire perf session under Wayland, but the
script `gnome-shell-perf-tool` does not spawn `gnome-shell` as a Wayladn
compositor, so that fails as well.

Add a `--wayland` option to `gnome-shell-perf-tool` so that it can
optionally 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
```

Alternatively, for testing purposes, it can also be started nested with:

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

Closes: https://gitlab.gnome.org/GNOME/gnome-shell/issues/2139
https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/941