tweak back-button

use a simple string instead of a view:
- when using a view, that is always displayed completely,
  which, in turn, results in longer chat-title being moved out center.
- when using a string,
  longer titles result in the string being limited to an back-icon only,
  titles stay centered that way (and there is more room for longer titles)

deltachat-ios/Controller/ChatListController.swift

@@ -169,9 +169,7 @@ class ChatListController: UITableViewController {
     // MARK: - setup
     private func setupSubviews() {
         emptySearchStateLabel.addCenteredTo(parentView: view)
-        let backButton = UIBarButtonItem()
-        backButton.title = String.localized("pref_chats")
-        navigationItem.backBarButtonItem = backButton
+        navigationItem.backButtonTitle = String.localized("pref_chats")
     }
 
     @objc