add campaigns

levels should be groupable to form a campaign.
campaigns can also contain sub campaigns.

levels and campaigns also need:

• identifier (level already has this)
• name (levels current name is used as identifier, TODO split to identifier and name)
• description
• enter dialog (when first played this dialog is shown) (optional)
• finish dialog (when (first?) finished this dialog is shown) (optional)
• requirements (a set of identifiers that need to be completed first)
• state (locked -> unlocked -> started -> finished)