Addressing review comments

 - enough with the gets already! :-)
 - new style boxing and enums
 - more idomatic var and method names
MissionItemFieldType enum -> NS_ENUM
Applying review recommendations to additional code
Removed a couple dozen warnings
Further review recommendation adoptions

 - regarding recent obj-c literals / declarative syntax
Merge branch 'quick-warning-pass'
auto synthesize properities in MissionItemField

- more idomatic method names and args
  for init methods in MissionItemFields
Merge branch 'master' into igcs-19-save-load-mission