loading the child asynchronously from server

@fritz-c thanks for the very great component. We want to know how we can load the children’s child asynchronously when the expand button is clicked.

Initially, we want to send the UI just the first depth and as the user clicks on the expansion button we want to pull more data. We sending column hasChildren boolean for a placeholder for asynchronous data. So when the expansion is clicked it will pull additional node data from the backend. Let me know what I need to tweak to get there.

       treeData: [
        {
          "title": "development",
          "isDirectory": true,
          "hasChildren": true,
          "children": [
            {
              "title": "examples",
              "isDirectory": true,
              "hasChildren": true,
            },
            {
              "title": "private",
              "isDirectory": true,
              "hasChildren": true,
            },
            {
              "title": "projects",
              "isDirectory": true,
              "hasChildren": true,
            }
          ]
        }
      ],

[megaboich]

I have created another PR to include example of lazy-loading nodes and also bug fix. Actually it was working quite stable, on React 15 for a while, but now it seems that React 16 is a bit harsh about unhadled exceptions, so it was not stable when you do drag and drop over not yet loaded nodes. So in PR I tried to avoid throwing an exeption in some cases and instead handling undefined result was added.

[veeramarni]

@fritz-c

We have created this PR for lazy loading to work. Please review and let us know.

https://github.com/fritz-c/react-sortable-tree/pull/214