monitor: improve heuristic to determine display output name

Under some circumstances, for example when the display controller driver
doesn't report back the correct EDID, or under VirtualBox, Mutter
returns suboptimal strings for an output display name, leading to funny
labels like 'Unknown 0"', or '(null) 0"' in the Settings panel.

This commit improves our heuristic in three ways:
- we now avoid putting inches in the display name if either dimension is
  zero
- we use the vendor name in case we're not able to lookup its PnP id
  from the database. Previously we would have passed over '(null)'
- as a special edge-case, when neither inches nor vendor are known, we
  use the string 'Unknown Display'

Finally, we make the combined vendor + inches string translatable, as
different languages might want to move the size part of the string to a
position different than the end.

https://bugzilla.gnome.org/show_bug.cgi?id=721674