Scene should derive from Task

[eoyilmaz]

User Story

As a developer using the Stalker library I want the Scene class to be deriving from the Task class so that it is possible ot use it in Task hierarchies along with Asset, Shot and Sequence.

Acceptance Criteria

1. Derive the Scene class from Task.
2. Add Alembic migration script for this change.

Original Description

The data that Scene entity is representing is commonly required in project hierarchies. But in the current implementation the Scene can not be directly used in Task hierarchies because it is not deriving from the Task and generally a Task with the type attribute is set to a Type object with the name attribute value of Scene is used and because we have a Scene class it creates a clutter.