sticky search result headers

let the headlines "123 chats", "234 contacts", "345 messages" not scroll away.

this makes the context of swiping action even a bit clearer,
targets #1869

deltachat-ios/Controller/ChatListController.swift

@@ -93,7 +93,7 @@ class ChatListController: UITableViewController {
         self.dcContext = dcContext
         self.dcAccounts = dcAccounts
         self.isArchive = isArchive
-        super.init(style: .grouped)
+        super.init(style: .plain)
         DispatchQueue.global(qos: .userInteractive).async { [weak self] in
             guard let self = self else { return }
             self.viewModel = ChatListViewModel(dcContext: self.dcContext, isArchive: isArchive)