Box and derived type should be immutable

[Orace]

Box and derived type should be immutable.
This property setters and methods should only be called in the constructor / at init:

• Add
• Source
• Foreground
• Background
• Italic
• Width
• Height
• Depth
• Shift

[ForNeVeR]

Yep, I approve this. We've been having a similar problem with Atom type, and that's been resolved recently (with C# records). I think a similar approach should work for boxes?

Keep an eye on how selected boxes highlighting works, though. I remember something weird about that.