Reviewed-by: Trust Me

In CppTools, keep cpp file settings in plugin and settings page
via shared pointer and apply to mime DB in
extensionsInitialized() (after CppEditor has loaded
and registered the mime types).

Task-number: 252299

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

It is now possible to choose between case-sensitive or case-insensitive
completion. Also, you can now disable the automatic insertion of braces.

To match the class name, CppToolsPlugin.