Shortcuts: Remember them even for plugins that are not loaded

This also fixes the issue that shortcuts of designer integration gets
lost if it doesn't get initialized.

Task-number: QTCREATORBUG-4264
Task-number: QTCREATORBUG-11399
Change-Id: Ieb4fe5fd345e69aa62403e4578fa6028400aff6f
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>