[LightDM] launch xvfb if no monitor detected

[Adam Nielsen]

> It looks like some combination  of 'headless' (no monitor)
> and an install script to answer all the questions?
> https://serverfault.com/questions/21255/headless-linux-install

You probably won't be able to run a headless machine because they don't
run a GUI.  This means they can't be used to run GUI programs like major
web browsers.  True, you could restrict yourself to console only
software, but console web browsers for example, tend to perform poorly
on mainstream web sites which only ensure compatibility with a couple
of popular browser engines.

Running a GUI, so that you can run a mainstream web browser along with a
screen reader, is probably going to provide a better experience.  I am
only guessing though, as I don't have any experience myself in this
area.

Cheers,
Adam.