WIP changes to mission editor. Changes summary: - Created new TutorialMission type, removed some fields that were no longer needed and moved some things pertaining only to tutorial missions to these. Updated tutorial mission assets to be of this type. - Renamed some fields on BaseMission and added tooltip descriptions. - Removed simple mission provider field from missions. Missions which belong to a simple mission provider are now defined on the provider. - Added FilteredEnum attribute which allows for the inspector to omit drawing one enum value and for defining a new default value for the field on int-backed enums.