Kits: Fix saving of kit data.

Make sure to not save in the KitManager destructor: Depending on how
creator is shut down (e.g. auto-shutdown after running plugin tests)
the other Managers may already be gone and then the Kit data will be
damaged.

Change-Id: Ieca06b8ac979024a90cb3b7d5e41f4506b4480bd
Reviewed-by: Daniel Teske <daniel.teske@digia.com>