boxes/views: Adopt urwid_readline for all editors.

The panel search boxes were the only remaining editors
not using urwid_readline.ReadlineEdit.

Author: Niloth-p

zulipterminal/ui_tools/boxes.py

@@ -964,7 +964,7 @@ def keypress(self, size: urwid_Size, key: str) -> Optional[str]:
         return key
 
 
-class PanelSearchBox(urwid.Edit):
+class PanelSearchBox(ReadlineEdit):
     """
     Search Box to search panel views in real-time.
     """

zulipterminal/ui_tools/views.py

@@ -682,7 +682,7 @@ def update_user_list(
             user_list is not None and search_box is None and new_text is None
         )  # _start_presence_updates.
 
-        # Return if the method is called by PanelSearchBox (urwid.Edit) while
+        # Return if the method is called by PanelSearchBox (ReadlineEdit) while
         # the search is inactive and user_list is None.
         # NOTE: The additional not user_list check is to not false trap
         # _start_presence_updates but allow it to update the user list.