Adjust $ID instead of adjusting the instructions

[michitux]

It might be worth an experiment to simply adjust $ID to the included page instead of adjusting every single link, media reference, plugin instruction, ... The advantage would be that all relative links work and a lot less link fixing would be needed. The disadvantage might be that plugins that use external data storage would write or read the data of the included page which might lead to inconsistencies but could also have advantages as quite often it is not wanted that this data is stored for the including page.

This could be easily implemented in the include syntax part but then this wouldn't work for other plugins like the blog plugin that use the include plugin as helper plugin. A more robust solution might be to add this functionality to the wrap syntax that is dynamically added by the include plugin.

[michitux]

A note for anybody who wants to work on this: This change probably won't be as good as I originally thought it was. I tried implementing this some time ago and had to notice that this creates many problems. Unfortunately I can't remember what these problems were exactly. Nevertheless this shall be a warning to first check if this really works and doesn't break everything.