UI: not all subclasses of CopyableLabelTableViewCell have a detailTextLabel, so generalize

WireGuard/WireGuard/UI/iOS/CopyableLabelTableViewCell.swift

@@ -6,6 +6,10 @@ import UIKit
 class CopyableLabelTableViewCell: UITableViewCell {
     var copyableGesture = true
 
+    var textToCopy: String? {
+        fatalError("textToCopy must be implemented by subclass")
+    }
+
     required init?(coder aDecoder: NSCoder) {
         fatalError("init(coder:) has not been implemented")
     }
@@ -41,7 +45,7 @@ class CopyableLabelTableViewCell: UITableViewCell {
     }
 
     override func copy(_ sender: Any?) {
-        UIPasteboard.general.string = self.detailTextLabel?.text
+        UIPasteboard.general.string = textToCopy
     }
 
     override func prepareForReuse() {

WireGuard/WireGuard/UI/iOS/TunnelDetailTableViewController.swift

@@ -311,6 +311,10 @@ class TunnelDetailTableViewKeyValueCell: CopyableLabelTableViewCell {
         set(value) { detailTextLabel?.text = value }
     }
 
+    override var textToCopy: String? {
+        return self.detailTextLabel?.text
+    }
+
     override init(style: UITableViewCellStyle, reuseIdentifier: String?) {
         super.init(style: .value1, reuseIdentifier: TunnelDetailTableViewKeyValueCell.id)
     }

WireGuard/WireGuard/UI/iOS/TunnelEditTableViewController.swift

@@ -441,6 +441,10 @@ class TunnelEditTableViewKeyValueCell: CopyableLabelTableViewCell {
         valueTextField.spellCheckingType = .no
     }
 
+    override var textToCopy: String? {
+        return self.valueTextField.text
+    }
+
     required init?(coder aDecoder: NSCoder) {
         fatalError("init(coder:) has not been implemented")
     }