Fix smoothly an `Appearance` property creates _many_ undo/redo entries

[adamkewley]

• When a user edits a model component with an Appearance property
• By (e.g.) opening the color editor and dragging their mouse over the various colors
• Every single drag point behaves as a "full model edit" and will trigger full recomputation (finalization, undo/redo storage, etc.)
• Which is very slow on on some computers - esp if editing a complex model
• And it fills the undo/redo buffer with many (tens, hundreds) of entries for each shade of color the user dragged through

The ideal behavior is:

• User edits color and can see the color changing in the UI
• User lets go of mouse
• Final color is committed to the model

But an ok intermediate is

• User edits the color but can't see it changing
• User lets go of mouse
• User sees new color and it is committed to the model

Which at least limits the undo/redo damage to "once per click"