So managing the various resources of this game has got too time consuming - I thought it would save time in the long run to write a resource editor. Also, I'd like this game to one day be mod-able by the users, so I'd have to write an editor eventually.
Currently the editor only allows you to add new planes, but eventually it will be used to define ground units too, maps, and also game scenarios such as campaigns, etc.
Here is a screenshot of a new plane being added:
...and here is another new plane having its sprite bounds defined: