Removal of some old demos that are no longer required.

See merge request thesmog358/tbblue!23

The CP/M system files are now freely distributable (see
docs/licences/CPM3-license.txt).

To take advantage of this, the old method of initialising CP/M from a
cpmbase.p3d disk image plus downloaded system files has been replaced.

The CP/M system files are now present in C:/NEXTZXOS/CPM. When CP/M is
first started, a new CPM-A.P3D image is created (if necessary) and the
files are automatically imported into it. The import also occurs if the
user has manually created a CPM-A.P3D image.

Dot command sources have also been reorganised.