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>

Alternative implementation of the indexer. It is a little bit slower, but I'm going to make it faster.
Unfortunately the refactoring of the indexer is kind of necessary. I need to merge the new binding pass.

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)

replace ExtensionSystem::PluginManager::instance()->getObject<Core::ICore>() by Core::ICore::instance()

forward declaration is sufficient.