Add a new compulsory file that identifies DCSC format collections

[delphidabbler]

At present there is no defined way for a program to detect a valid DCSC collection. Programs have to test for the presence of REQUIRED files. According to the spec as at v2.3.x these are:

• VERSION
• LICENSE
• CONTRIBUTORS
• TESTERS

Perhaps surprisingly the following files are not explicitly required by the specification:

• categories.ini
• LICENSE-INFO

It would simplify matters greatly if another REQUIRED text file, say ID was introduced that contained a single line that is a unique identifier for this file format. This could be a GUID. The content of ID would never vary, and the GUID would be published in the spec.

This shouldn't break backward compatibility if the file was only REQUIRED from the version where it was introduced, which would have a minor version number bump.

Programs could then proceed as follows:

if file ID is present then
    read ID and check its content against the published GUID
else
    perform legacy chacks for the presence of files

Programs could then go on to check the version number in VERSION to test for compatibility.

[delphidabbler]

An alternative that would save having to read the ID file to get the UID would be to have an (optionally) empty file named with the UID, for e.g. CE506DF7-D1CB-4FCA-8B49-15F5C54FE65C or CE506DF7D1CB4FCA8B4915F5C54FE65C

[delphidabbler]

This is making backward compatibility too unwieldy. It's time for v3 I think.