De/serialize render tree data more efficiently

[blaugold]

Currently, render tree data is serialized as a recursive JSON structure. This is expensive to both serialize and deserialize.

Instead, each RenderObjectData should be serialized and appended to a string, separated by a delimiter. Each RenderObjectData needs to be serialized with a unique ID, which is used to refer to it in child lists. With this, the tree structure can be reconstructed during deserialization.