Allow changing color and position of the postcard name and contents

### I am planning on implementing this myself

Yes

### Describe your request

Self-explanatory, allow mappers to change the postcard recipient text color and position, as well as the postcard content position.

Could be neat to convert this into a separate YAML object like so:
```yaml
Postcard:
Texture: "postcard texture" # relative to the Gui atlas
SoundID: "fmod event" # would be an alias to the existing PostcardSoundID YAML field?
NameColor: [x, y] # color of the save file name
NamePosition: [x, y] # position of the save file name
ContentPosition: [x, y] # position of the postcard contents
```

### Additional context

Expands upon #1056. This would let mappers implement dark postcard textures as well as non-standard postcard formats.