Viewing entities in Entity tree

[nelliputkonen]

• [ ] 1) In addition to viewing the entities under a certain entity class by toggling the arrow next to entity class, add a new table within Entity tree window that lists all entities of the class (or classes) that the user selects. This table could be located to the right of the entity class list (the Entity tree would then be two lists: the left showing list of entity classes and the right showing list of entities (dynamically changing based on which class or classes are selected).

• [ ] 2) To the newly created entity list, add ordering/filtering option: Add option to order or filter Entity tree by 'entity group' (see issue https://github.com/spine-tools/Spine-Toolbox/issues/2788 for adding 'entity group' to 'Add entities' window). Currently entity tree is ordered only alphabetically, but using 'entity groups' would allow for more customized ordering. Probably simplest still is that you can order alphabetically by 'entity group' or filter only the groups you need.