Added Grid Loading Indicator sample and fixed Tgrid

[tishko0]

Grid's Toolbar Exporting Indicator sample was missing.
Also fixed TreeGrid since the data loop was pushing unique id's to the data array, reduced data lenght since generating 3 million entries was taking too long.

[MarielaTihova]

Registering IgrGridModule is no longer needed

[MarielaTihova]

We don't need refs for grid and toolbar

[MarielaTihova]

Since we are using React it would be nice to put this data setup into a useEffect and then pass the data as state. FOr example

const [localData, setLocalData] = useState([]); 
 useEffect(() => { 
    const data = localData;
    for (let i = 0; i < 10000; i += 3) {
      for (let c = 0; c < athletesData.length; c++) {
        data.push(athletesData[c]);
      }
    }

    setLocalData(data);
  }, []);

[MarielaTihova]

Instead of using refs to manipulate the dom you can just use this

<IgrGridToolbar key="toolbar" showProgress={showProgress}>
and set up the progress indicator visibility like this using component state

  const setupProgressVisibility = () => {
    setShowProgress(true);

    setTimeout(() => {
      setShowProgress(false);
    }, 5000);
  }

and then pass this function to the button
<IgrButton key="btn" onClick={setupProgressVisibility}>

[MarielaTihova]

Please apply all of the changes requested for the flat grid here too!