Add class_ getter for type

[ufrshubham]

Since Tiled 1.9, the Type property for objects has been changed to Class by default. This PR modifies the parsing logic to first look for class and then type before using the default (empty string) value. We need to look for both keys because Tiled has also provided a compatibility option in project properties using which, users can force Tiled to write out Class property as type in exported files. I've also added a getter called class_ which returns type so that it will be easier for users to find this property.

Details about this change is mentioned in Tiled 1.9 release notes under section "Unified Custom Types" : https://www.mapeditor.org/2022/06/25/tiled-1-9-released.html