Allow period and colon in section ids.

[trollkotze]

A verbose explanation why including colons and periods in section IDs should be allowable and does not cause problems can be found in this 3 year old issue: #2580
Running a DokuWiki with this small modification since 3 years, allowing '.' and ':' in section ids and anchor links to them etc. has caused no problems in all this time.

[Klap-in]

I think it is logic to allow these characters.

Could you please extend the unit tests?
https://github.com/splitbrain/dokuwiki/blob/master/_test/tests/inc/pageutils_sectionid.test.php

Should we consider (breaking) backward compatibility? Some existing links with these characters will break..

[trollkotze]

@Klap-in Sorry, I'm not familiar with your unit tests so am not sure if I understand all around correctly.
Since the change in functionality here is just to allow '.' and ':' in section ids, I guess one would just change the test sequences from

            [['A', 'A', 'A1']],
            [['A', 'A1', 'A']]

to something including colons and dots? I would appreciate if you could take this up since you probably understand better what to test for.