Move preview_text method to XorgProfile instead of subclasses implementing it

[lavafroth]

This is a rather simple suggestion relating to the desktop profiles in archinstall/default_profiles/desktops.

We might move preview_text method to XorgProfile class in archinstall/default_profiles/xorg.py instead of subclasses implementing the same method. This can dry up some code as new desktop profiles will most probably want to stick to a bunch of packages due for installation.

Of course, if the desktop profiles choose to, they may always override this behavior by implementing the method.

[svartkanin]

I think that's a good change, feel free to raise a PR