Use NSToolbarItemGroup

[sindresorhus]

When we can eventually target macOS 10.15 in the far future, we could use NSToolbarItemGroup instead of placing a segmented control in the toolbar.

https://developer.apple.com/documentation/appkit/nstoolbaritemgroup/selectionmode/selectone
https://developer.apple.com/documentation/appkit/nstoolbaritem/3237224-isbordered
https://developer.apple.com/documentation/appkit/nstoolbaritem/3237225-title

All the new APIs:

Added NSToolbarItemGroupSelectionMode
Added NSToolbarItemGroupSelectionModeSelectOne
Added NSToolbarItemGroupSelectionModeSelectAny
Added NSToolbarItemGroupSelectionModeMomentary
Added NSToolbarItemGroupControlRepresentation
Added NSToolbarItemGroupControlRepresentationAutomatic
Added NSToolbarItemGroupControlRepresentationExpanded
Added NSToolbarItemGroupControlRepresentationCollapsed
Added +[NSToolbarItemGroup groupWithItemIdentifier:titles:selectionMode:labels:target:action:]
Added +[NSToolbarItemGroup groupWithItemIdentifier:images:selectionMode:labels:target:action:]
Added NSToolbarItemGroup.controlRepresentation
Added NSToolbarItemGroup.selectionMode
Added NSToolbarItemGroup.selectedIndex
Added -[NSToolbarItemGroup setSelected:atIndex:]
Added -[NSToolbarItemGroup isSelectedAtIndex:]

http://codeworkshop.net/objc-diff/sdkdiffs/macos/10.15/AppKit.html