[Feat]Separate dropdown/tab for completed tasks

**Is your feature request related to a problem? Please describe.**
User might not want to see their completed tasks along with the non-completed or pending tasks.

**Describe the solution you'd like**
A different tab for completed tasks so that the user easily is able to see its completed and pending tasks separately.

The issue does not name any files or tests. Start by locating the Flutter task-list screen and how completed versus pending tasks are represented; done means users can view completed tasks separately from pending tasks through a tab or dropdown.

Written by the indexing model from the issue text.