This adds a way for plugins to tell the cppmodel that for certain files,
the actual contents come from it, instead of reading from the disk.
(For texteditors the cppmodel does that automatically.)

Reviewed-By: Roberto Raggi <roberto.raggi@nokia.com>

Especially when several error markers exist, the flashing this causes
while editing isn't really nice.

Reviewed-by: Roberto Raggi

(cherry picked from commit 9539bb2b)

It used to be in CppTools, but since the hover handler only makes sense
in the context of the C++ editor, this is a better place.

This change removes a number of wrong usages of the CppModelManager::documents()/document().