Make folding work and simplify it; use proper syntax group names.

- Don't use separate collapsibleSectionStart/End groups; instead define a fitnesseCollapsibleSection and use matchgroup=... there.
- fitnesseBang must contain fitnesseCollapsibleSection for the folding to work.
- Remove the useless buffer-local variable and clean up the contradicting :syntax sync stuff.
- Prefix all syntax groups with the syntax name; define additional highlight links as needed.
Include XML syntax highlighting.
Include XML syntax highlighting with :syntax include.

This avoids matching xmlError in areas that are *not* XML. Copied over the fragment that defines the outermost XML scope and use that here; the rest is automatically handled by the containment.