Add multi-select for list

[dweymouth]

Description:

• Adds new enum widget.SelectionMode (values: SelectionSingle, SelectionMultiple, SelectionNone)
  • This only effects the tap/key handling, not the programmatic APIs
• Adds tap/key handling for multi-select, respecting SelectionMode setting
• Adds a few new APIs related to multi-select
  • SelectOnly(ListItemID)
  • SelectAll()
  • SetSelection([]ListItemID)
• The list.Select API now adds to the selection (which is what the documentation said), rather than replacing it

TODO/ Concerns:

• tests
• This uses a slice of IDs to keep track of selection, and creates a new selection slice on any unselect operation. This makes thread-safety fairly easy, but comes with some performance tradeoffs:
  • Repeatedly unselecting items causes a lot of memory churn (UnselectAll is implemented efficiently)
  • setupListItem is now O(n) in number of selected items. This is a potential concern
  • Notifying onSelected/onUnselected callbacks is O(n^2) in operations that select multiple IDs at once (SelectAll, SetSelection)

Fixes #2575 (for list)

Checklist:

• Tests included.
• Lint and formatter run with no errors.
• Tests all pass.

Where applicable:

• Public APIs match existing style and have Since: line.