Make Core independent from QtHelp

"Help" is a separate plugin, but to avoid a dependency of "everything" on the help plugin, we have a help manager in Core, and that actually performs the documentation registration.

That is ugly. Possible modifications for a better separation:

1. Keep the help manager in core, but let is just manage the list of files. Do actual registration in Help, communicate through signals etc.
2. Move the help manager to help, and use a build-time-header-only and optional runtime dependency for the plugins that actually do want to register documentation. If these are not so many after all.