Allow users to calibrate the pen input of wacom devices

[JMoerman]

Problem

It currently isn't possible to calibrate the Wacom pen input to a position on the screen. On windows, Mac OS (have tried it once in the OS-X days) and on GNOME shell it is possible to calibrate the device by touching a cross or dot in 3 to 4 corners of the screen. Without this possibility the input may have an offset, may be heavily distorted (different height to width ratio of input device and screen).

Proposal

Provide a way for users to calibrate their Wacom devices. Users should be able to calibrate every Wacom-screen combination (related: #19). When starting the calibration process, all inputs from the Wacom device should be intercepted and handled*, and a full screen window should provide calibration points to touch with the pen. These points should be used to map the input of the device to absolute positions on the screen.
*Should an input land outside the calibration area it must be handled else it may not be possible to fix badly calibrated setups.

Prior Art

Wacom's drivers for the proprietary OSes come with utilities to calibrate Wacom devices.
Gnome shell has a control panel screen with an option to calibrate Wacom devices similar to what is possible on Windows.