Add a new long-form detailed task description

[JeanMertz]

Currently, the task description is used as a short-form explanation of the task when showing a list of tasks:

When opening a task, that same description is shown at the top:

In this last case however, you sometimes want to provide additional minutiae details that aren't important enough to clutter the task listing (or are too long to list), but are important enough to show somewhere.

This is what a long-form description could provide.

• If one is provided, it is shown in the task details window
• If none is provided, the regular short-form description is shown

I'm not sure on the naming yet.