SP-30: Steps for GNOME text scaling factor = 0.1 instead of 0.5

The GNOME Tweak Tool uses steps of 0.1 for text scaling.
Merge remote-tracking branch 'cstrobbe/patch-5'

* cstrobbe/patch-5:
  SP-30: Steps for GNOME text scaling factor = 0.1 instead of 0.5