Respect physical display sizes in xfce4-settings insofar as practical

  Prior to this change, on startup, xfsettingsd would always set the nominal
  physical dimensions of the (typically sole) virtual screen to correspond to
  265-micron pixel pitch (96 dpi). However, as physical displays with much
  smaller pixel pitches become common, this doesn't make much sense when all
  of the physical devices attached have significantly smaller pitches (even
  if they don't all match) and especially problematic when there is a single
  output (a relatively common case) with a very different pixel pitch.
  In this case, the physical dimensions are very meaningful, and altering
  them can lead to such symptoms as poorly chosen font sizes and widely varying
  font sizes among different applications.

  This commit always respects any physical dimensions reported by xrandr in
  the case of a single output and makes a best effort to patch together a
  ballpark reasonable physical dimension in the case of multiple outputs. (In
  any case, the computed effective physical dimensions will always reflect
  a pixel pitch that is between the smallest and largest pixel pitches
  reported by xrandr for any outputs.)

  Resolves (XFCE gitlab) issue #34 (formerly XFCE bugzilla issue 10633 and
  its various duplicates).
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