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().