Fix sections and section editing

Another go at fixing this plugin

Compatibility with different renderers, e.g. qc plugin uses an array

Section editing is only available to users who can write the page. User
who can write the page have access to all data in it, so hiding content
from them with ifauthex is useless in the first place.

For proper handling, we might need to overwrite the xhtml renderer and
tune a lot of internal variables and properties.

We can simply add baseonly as allowed and let headers do their own
thing.

The plugin gets reused for multiple renderers, but we need independend
references for each one. So we put them into the meta array of the
renderer.

For now we ignore the handling of sections.

this is a passing test that probably should not pass.

Solution from wrap plugin

This changes the way how content is hidden from the user by diverting
the renderer doc to a sacrificial space. This way all rendering outputs
to this space and is simply thrown away after the fact.

This has two advantages:

1) no separate renderer processes need to be instantiated (less resource
hungry)
2) section edit byte positions stay correct

Today I'm a bit sick.

Weirdly this does not trigger during testing.
PHP scope magic?

In this way it is possible to test the expression value.

See comment.

There are some weird quirks in DokuWiki.
1. The regex syntax for the Entry/Exit pattern is somewhat reduced.
2. Using a substitution type does not work.

Replaced with functions which wrap over static variable.

Directory name must match the plugin.