Draft: Add tablet area configuration
This adds the configuration option to define a rectangle that serves as an input area on external tablets such as an Intuos.
The intention behind this is to make this input area behave as if it was the only physical input area on this tablet with libinput emulating proximity events as required for where the tools moves in and out of this area.
This could also be achieved with the existing calibration setting but area configuration is not calibration and we don't want to expose other side-effects of the matrix (e.g. scaling and rotation) for these devices.