Add tablet area configuration

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.

Part-of: <https://gitlab.freedesktop.org/libinput/libinput/-/merge_requests/1013>