Tidy up scatter code

[dstansby]

This is some misc tidying to the scatter code:

• Use Optional instead of Union[None, ...] in typing
• Remove a redundant __init__()
• Always add the selection GUI to the property scatter (we can change to make this optional later, but for now I think keeping code simpler is better)

@kevinyamauchi could you take a look at this and see if it all looks okay?

[kevinyamauchi]

This changes make sense to me! See my comment above for a fix for the update function. Happy to discuss further!