Support for `NSTableView` sections (aka group rows)

Checklist

[x] Reviewed the README and documents.
[x] Searched existing issues for ensure not duplicated.

Description

The NSTableView extension doesn't apply section changes and indices assume a single section.

Motivation and Context

Like the UITableView, NSTableView supports sections (called "group rows" in AppKit). It would be great if they would just work in combination w/ ArraySection's.

Proposed Solution

In NSTableView the sections (group rows) are part of the collection feed to the tableview. E.g.

let source = [
    ArraySection(model: "Section 1", elements: ["A", "B", "C"]),
    ArraySection(model: "Section 2", elements: ["D", "E", "F"]),
]

Is presented to NSTableView as:

[0] "Section 1"
[1] "A"
[2] "B"
[3] "C"
[4] "Section 2"
[5] "D"
...

It's not obvious that this isn't supported. Maybe AppKitExtension.swift should have

            assert(changeset.sectionInserted.isEmpty, 
                        "srz, section updates not yet supported on AppKit!")
...

in the reload.

Not quite sure about the best way to implement this. The reload should have all the grouping information necessary. Essentially there would need to be a separate step adjusting the indices to the flat ones, while applying the changes.

Sample TV datasource/delegate on top of `ArraySection`

extension AppDelegate: NSTableViewDelegate {
  func tableView(_ tv: NSTableView, isGroupRow row: Int) -> Bool {
    switch test.data[flat: row] {
      case .some(.model): return true
      default: return false
    }
  }
  func tableView(_ tv: NSTableView, viewFor tc: NSTableColumn?, row: Int)
       -> NSView?
  {
    let label = NSTextField(labelWithString: test.data[flat: row]?.stringValue ?? "-")
    return label
  }
}
extension AppDelegate: NSTableViewDataSource {
  func numberOfRows(in tableView: NSTableView) -> Int {
    test.data.flatCount
  }
}

final class SectionedTest {

  var data = [ ArraySection<String, String> ]()

  func applyNewData(_ newValue: [ ArraySection<String, String> ],
                    on tableView: NSTableView)
  {
    let changeset = StagedChangeset(source: data, target: newValue)
    tableView.reload(using: changeset, with: .effectFade) { newValue in
      self.data = newValue
    }
  }
}

enum Row {
  case model(String)
  case element(String)
  var stringValue: String {
    switch self {
      case .model(let s), .element(let s): return s
    }
  }
}
extension Collection where Element == ArraySection<String, String> {

  var flatCount : Int {
    reduce(0) { $0 + 1 + $1.elements.count }
  }

  subscript(flat index: Int) -> Row? {
    var cursor = 0
    for section in self {
      if cursor == index { return .model(section.model) }
      cursor += 1
      if cursor > index { break }

      let offset = index - cursor
      assert(offset >= 0)
      if offset < section.elements.count {
        return .element(section.elements[offset])
      }
      cursor += section.elements.count
    }
    assertionFailure("index out of range \(index) \(count)")
    return nil
  }
}

ra1028 / DifferenceKit