Allow users to calibrate the pen input of wacom devices

## 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: https://github.com/elementary/switchboard-plug-wacom/issues/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.