Show a confirmation dialog when deleting a message

[RemiBardon]

When the user clicks "Delete…" in a message's actions menu, we do not present a confirmation dialog yet. I wrote "Delete…" to suggest that the action has more steps, which could let the user cancel their action. It is currently misleading, and we have to choose between:

1. We want to show a confirmation dialog
   • In this case, useful resources are:
     • SwiftUI's confirmationDialog(_:isPresented:titleVisibility:actions:)
     • pointfreeco/swiftui-navigation .confirmationDialog overloads
     • pointfreeco/swiftui-navigation case studies: 02-ConfirmationDialogs
2. We don't want to show a confirmation dialog (bad UX)
   • Replace "Delete…" by "Delete"
   • Implement a "Recently deleted" feature, or add support for UndoManager (see Global undo/redo #117)
   • Show some kind of hint (alert, onscreen tutorial…) explaining how to undo this action
     • We can show this a limited number of times (once, once every t time, until the user uses the feature…) not to bother the user too much