Reduce payload

[falkoschindler]

This PR tries to reduce the websocket payload when updating elements.

• exclude empty fields from the element dictionary
• exclude the "key" prop
• exclude the "template" field in the slot dictionaries?
• reduce the default slot from

  "slots": {
    "default": {
      "template": null,
      "ids": [
        213,
        214
      ]
    }
  }
  

  down to

  "children": [213, 214]
  

• remove the "id" from the dictionary values? (they are repeated as the dictionary keys)

[falkoschindler]

For pushing a line to the new ui.log element (see PR #2686), this reduces the payload from

{
  "211": {
    "id": 211,
    "tag": "div",
    "class": [
      "nicegui-log",
      "w-full",
      "h-20"
    ],
    "style": {},
    "props": {
      "key": 211
    },
    "text": null,
    "slots": {
      "default": {
        "template": null,
        "ids": [
          215,
          216
        ]
      }
    },
    "events": [],
    "component": null,
    "libraries": []
  },
  "216": {
    "id": 216,
    "tag": "div",
    "class": [],
    "style": {},
    "props": {
      "key": 216
    },
    "text": "09:33:26.3",
    "slots": {
      "default": {
        "template": null,
        "ids": []
      }
    },
    "events": [],
    "component": null,
    "libraries": []
  }
}

down to

{
  "211": {
    "tag": "div",
    "class": [
      "nicegui-log",
      "w-full",
      "h-20"
    ],
    "children": [
      215,
      216
    ]
  },
  "216": {
    "tag": "div",
    "text": "13:59:20.5"
  }
}

[rodja]

Super cool!