fix: Object layer parsing for JSON format

[JochenHeizmann]

Description

When loading and parsing JSON Format, objectGroup layers wheren't parsed correctly, which resulted in empty entry.

The main problem was that in JSON Format it is called "objects" instead of "object" like in XML. Here is a snippet of a JSON file exported with Tiled 1.8.2:

         "draworder":"topdown",
         "id":8,
         "name":"obj",
         "objects":[
                {
                 "height":8,
                 "id":1,
                 "name":"start",
                 "rotation":0,
                 "type":"",
                 "visible":true,
                 "width":8,
                 "x":64,
                 "y":160
                }, 
                {
                 "height":8,
                 "id":2,
                 "name":"gem",
                 "rotation":0,
                 "type":"",
                 "visible":true,
                 "width":8,
                 "x":96,
                 "y":40
                }, 

Breaking Change

• Yes, this is a breaking change.
• No, this is not a breaking change.