Reference objects as NodePath rather than Tiled id

[bitbrain]

Compatible with Tiled Version: 1.9.0

Description

Within Tiled, objects can be referenced like so:

However, within Godot the meta field shows up as Tiled integer id. This pull request changes this behaviour to instead populate the node path of the referenced object for easier lookup.

[bitbrain]

One thing I noticed with this is that it is not recursive - when referencing paths on custom attributes through custom classes, e.g.:

However, I cannot implement that before https://github.com/vnen/godot-tiled-importer/pull/158 is ready, as currently reading these attributes is impossible.