Add set of properties to doc::UserData

[dacap]

Related to #3625

Add a set of sets of typed-properties (including points) to doc::UserData depending on extension ID (empty ID are user-defined properties, just like the current user data field).
Something like

std::map<std::string, std::map<std::string, std::any>> m_properties;

or

std::map<std::string, std::map<std::string, Variant>> m_properties;

?
Where Variant = std::variant<bool, int, std::string, gfx::Point, etc….> (these types will be limited to the .aseprite format, and probably will not be possible to extend it after the initial set of types)

[dacap]

@martincapello probably this issue is more important than the serialization first so the Lua API can be implemented (not sure if it does make sense to just push a first PR with just this change and then do an intensive test of .aseprite changes with automatic tests/etc.)