Add Backpack and Collectable properties to StoryItem

[birbilis]

Version: 0.6.0

It would be useful for making games if one could pick items along the way as they drag an item. E.g. they could pick some key

A Backpack item could also be Collectable so that one could use it to make sub-collections (like a sack/chest of coins - a treasure trove that one can collect etc.).

Tags of items they carry could be applied automatically as if they were tags of the item that is/has the backpack (probably in a nested way if collected items are backpacks that contain other collectable items). That way when you have an obstacle that only allows items with specific tag to pass-through, carrying the appropriate item could let you through (say a red door allows a red key - those are visual cues - underneath they'd share a common "RedKey" tag or similar)

When defining something as a Collectable could define its "weight" (an arbitrary integer number)
When defining someting as a Backpack could define its capacity, how much "weight" it can carry.
The weigtht could be set to 1 for example and set the Backpack capacity to 5 to mean it can carry up to 5 items.

User should be able to open the backpack somehow (right click maybe an item [hold down in touch mode]) in non-Edit mode (in Edit mode could do from the properties pane) to pick an item to drop at the current location of the backpack-enabled item. They could opt to drop some if they reach the max capacity of the backpack of a backpack-enabled actor/moveable StoryItem they're dragging around