Install mesa-utils-extra by default

Ubuntu 16.10 and 17.04 installed mesa-utils by default because libqt5gui5 at the time had Recommends: mesa-utils. mesa-utils provides these binaries:
glxdemo
glxgears
glxheads
glxinfo

For 17.10, Ubuntu plans to switch to GNOME on Wayland as default (or at least installed by default desktop). glxgears can run in GNOME on Wayland but it is running under XWayland instead of using native Wayland. mesa-utils-extras provides these binaries:
es2_info
es2gears_wayland
es2gears_x11
es2tri
(By the way, es2gears is symlinked to es2gears_x11).

So if a user wants to test whether Wayland is running successfully, they can try to run es2gears_wayland. If Wayland is not running, the app will not start.

Therefore, I propose that we install the minimal mesa-utils-extra by default as a fun troubleshooting tool. mesa-utils-extra is built from the same source as mesa-utils (mesa-demos) so all we need is to add this as a Recommends from ubuntu-desktop.

These are some other ways to check if you are running Wayland:
https://askubuntu.com/questions/904940/how-can-i-tell-if-i-am-running-wayland/968772#968772