Many singletons have their own instance() method, in which case there is
no need to use ICore::instance() to get access to them.

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

forward declaration is sufficient.

Details:  - Naming convention

Details:  - To make it conform to naming convention.

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.