☄ Move categories & entities via drag&drop

[mrfelipemartins]

Description

This PR aims to implement a Drag & Drop system to the Entity Tree allowing us to reorder the tree at will and move Entities between Categories safely.

Addressed issues

#14

Implemented Rules

Categories

• Cannot be dragged into themselves.
• Can be dragged into other categories or reordered within the same category or new category.

Entities

• Cannot be dragged into another entity.
• Can only be dragged into categories or reordered within the same category or new category.

If an entity's properties will change as a result of being moved to another category, a confirmation dialog will be presented to the user.

[bitbrain]

As mentioned on Discord, the API/Backend should be completely independent of any UI code. Instead of introducing concepts such as "tree" into the backend, introduce a method in the backend that handles moving entities/categories around in a "theoretical sense" and then the UI code accesses the Pandora singleton.

[bitbrain]

Great job!