Should still be ok on small screens though.
Suggested by Christian Hoenig.

Task:     - 242931

It was missing.

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

The previous interface wasn't sufficient to properly support the Apply
button, since it didn't separate applying the changes from the cleanup
phase.

Designer settings pages still need some fixing.

Fixes Apply button causing problems for certain plugins, for example the
QuickOpen.

Default shortcut is Ctrl+Shift+R now, but maybe there's something better
or more widely accepted. No menu entry yet.

Done based on new IQuickOpenFilter property, isHidden.

suggestion and request by the doc team.