Paper over array weirdnesses

[ben]

As per #43, it looks like most assets in the compendium have their data structured properly:

{
  abilities: [
    { enabled: true, description: "..." },
    { enabled: false, description: "..." },
    { enabled: false, description: "..." },
  ]
}

I.e., abilities should be an array. But others seem to have data that looks like this:

{
  abilities: {
    0: { enabled: true, description: "..." },
    1: { enabled: false, description: "..." },
    2: { enabled: false, description: "..." },
  }
}

abilities is an object with integer keys. This is probably left over from early development when I was still figuring out how to do editors and toggles, and honestly Foundry still sometimes screws this up.

This PR fixes it by using a custom handler for the checkboxes that will ensure the data ends up in the proper format. I'm also fixing a couple of other bugs with field edits on this sheet. Fixes #43.