Add support for custom classes in custom properties

[Frankfires]

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Tiled lets you create custom classes that can be used as property values:

The image above shows that I have two properties in my map: Spawn and a testing property named fffff, Spawn is a custom Vector2 class that I have created. When I try to access Spawn through the map's properties, it returns an empty string instead of a Vector2 containing its X & Y values as it should.

This issue restricts developers such as myself from managing their properties efficiently and in an organized manner