Item1038: Convert ExplicitNumberingPlugin

Move ExplicitNumberingPlugin to Foswiki name space.

Received following feedback from ThomasWeigert

However, I need to alert you of a problem with the current scheme that I discovered which I suggest that you tackle as part of the port. The explicit section numbering works as advertised, but it causes problems when creating anchors to sections explicitly numbered, as explained below....

Cheers, Th.

Given a section like this

Name of section

Anchors such as this

Name of section

do not work. The reason is that the ExplicitNumberingPlugin creates the anchors including the section number. E.g., in above example, the correct anchor might be

#2_3_Name_of_section

That is, of course, not great, as it would require changing the anchor whenever the section number changes.

Ready for testing I'm releasing this with a "Beta tag in the "Release" string. I've added code to exclude expanding text in <textarea> blocks, and tested with the EditChapterPlugin. It seems to be compatible now.