kcms/tablet: Start adding tests for the calibration tool
This is quite a math-heavy class, so it would be nice to start testing if it spits out the values we expect.
This makes a few changes:
- Changes the layout of CalibrationTool to not hold a reference to InputDevice, but take it as a function argument as needed. This is mostly to facilitate easier test writing.
- Document all of the public functions of CalibrationTool
- Add a new test under tst_calibrationtool for ensuring it generates an identity matrix under perfect conditions.
This should also make it easier to write more tests in the future for this tool.