Allow changing color and position of the postcard name and contents

[SnipUndercover]

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:

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.