Feature: calibration

[quozl]

The activity was written for the OLPC XO laptop, which had a known display size and resolution, so it was possible to present a ruler and have it always the best match to physical reality.

The activity makes no sense to use on other laptops or virtual machines, because the display size and resolution is different, or unknowable.

Add a calibration feature;

• use system information to guess the resolution, (some systems have display dimensions in EDID),
• present a calibration marking (not a ruler), and ask user to confirm with a physical ruler,
• do not display a ruler until calibration is complete,
• preserve the calibration in the activity instance directory,
• allow user to recalibrate at a later time (i.e. they did it wrong the first time, or they are using a different laptop with the same activity instance directory).