Multi-user selection painting

[matthew-carroll]

WORK IN PROGRESS

Multi-user selection painting was filed in #631.

Remote user selections need to be painted alongside the primary user's selection.

The big picture

Multi-user selection painting was achieved with the following changes:

• Add non-primary selection set to DocumentComposer, which logically represents all document selections other than the primary user's selection.
• Add a NonPrimarySelectionStyler to SingleColumnLayoutSelectionStyler, which is responsible for choosing colors for each non-primary selection.
• Change component view models to accept a list of StyledSelections instead of a single set of selection styles: ListItemComponentViewModel, BlockquoteComponentViewModel, ParagraphComponentViewModel, ImageComponentViewModel, and HorizontalRuleComponentViewModel.
• Change TextComponent to accept a list of StyledSelections instead of single set of selection styles. Also change TextComponents widget tree to use a SuperTextWithSelection.multi() instead of SuperTextWithSelection.single().

Experience with the changes

Making the changes was relatively straightforward. The only necessary re-wiring was the replacement of individual properties like selectionColor with a list of StyledSelections, each of which includes the selectionColor and the associated selection.

View model changes were monotonous and nearly identical. Each of the effected view models needed to have their constructor arguments changed to replace individual properties with the list of StyledSelection. The same change needed to happen to the view models' properties. The hashCode and == methods needed to be updated. The == methods needed the introduction of DeepCollectionEquality() to check list item equality.