Rearrangeable columns

[chucker]

Implements #154

Tried this in SwiftUI, but SwiftUI drag & drop on macOS is apparently quite limited. So, NSCollectionView instead.

• Disable wrapping on collection view items
• Implement drag & drop
• Pass window columns
• Need to make the previous menu item active again after exiting sheet
• Need to update menu items for existing column as well. For example, if the window previously had just one column, and one is added, we need to add Rearrange… to it.
  - [ ] Maybe: pass each column's aspect ratio as well?
• finish layout of view items
• Remove (obsolete) SwiftUI code. Leave for now so we can prototype the items.
• Apply change
• dragging is partially broken: for example, dragging from 0 to 1 doesn't seem to work