Default scale factor for common resolutions needed

Currently you are always in trouble when trying to connect a LoDPI projector to a HiDPI laptop.

If e.g. scale of 2 was chosen for laptop and then you connect an external LoDPI screen and select mirror, all the controls stay huge, but the scale should go down to 1 by default to avoid dealing with huge windows, which don't even fit on the screen.

Moreover, when external mirrored screen is disconnected, scale should be restored to 2 automatically to avoid user trying to find the tiny scale slider on a HiDPI screen.

There should be reasonable default scales for common screens also out of the box.
Right now, starting Ubuntu on 3200x1800 13" laptop has the initial scale of 1, forcing the user to find the scaling slider and operate it while it is 2mm big.

$ xrandr
Screen 0: minimum 320 x 200, current 3200 x 1800, maximum 8192 x 8192
eDP-1 connected primary 3200x1800+0+0 (normal left inverted right x axis y axis) 294mm x 165mm

This info should be enough to set scale=2 by default, and reset scale to 1 when resolution is 1920x1080 and below.

The problem is there with both 16.10 and 17.04

unity-control-center:
  Installed: 15.04.0+17.04.20170402.6-0ubuntu1