todo.txt: Reminders & Notifications for due date

#### General information

* **App version:** v2.1.x
* **System:** v9

#### Description
todo.txt format has support for custom key-value pairs. One common supported custom key-value pair is the `due:2019-12-25` tag for specifing the due date of a todo.
The todo.txt implementation at Markor (Stt) already supports and does parse custom tags.

This issue is about making use of the `Map` key value pairs that are already extracted. Out of that information with todo`s due dates, schedule notifications for the day of of due.
When opening a file with scheduled notifications, and a due date was removed - also remove the notification.
Please don't add android logic to SttCommander/SttTask. Parsing and determining due dats YES, but NOT like context/notificationChannel etc.

So TL;DR: Create reminders for todos.

Start by tracing how todo.txt custom key-value pairs and due dates are parsed, including the SttCommander/SttTask entry points mentioned in the issue. Keep Android notification concerns outside those components. Done means notifications are scheduled for todos with due dates and removed when a due tag is removed.

Written by the indexing model from the issue text.