Item & Filter ordering

[Brendonovich]

Currently, reordering is not supported and the only way to get the index of something is to check its position in its parent's list or ask OBS for it. This is undesirable since the parent's list only contains sceneify-controlled items, and excludes user-created items in OBS. Thus, more work must be done.

In the short term:

• Subscribe to reorder/reindex events and update a value on filters and items specifying their index
• Provide a reorder function that wraps the appropriate obs call

In the long term:

• Listen for addition/removal of user-created items and insert dummy objects into lists where necessary
• Make getting item index and performing reordering reflect the position of the item in its parent's array, rather than solely relying on OBS