Add TreeList "using remote data" demo/functionality

[nstoychev]

Here is how it can be achieved - https://stackblitz.com/edit/react-xchvii-rx7cfn?file=app%2Fmain.jsx.

Currently the TreeList cell component does not have built-in loading icon. Adding and positioning a loading icon does not look straightforward.

A loadingField prop of the TreeList could be introduced and the loading icon can be rendered instead of expand icon when necessary.

[nstoychev]

Requested again in Ticket ID: 1511547

[WissamProgress]

Also requested in Ticket ID: 1608447 where loading data on demand is needed

[filipKovachev]

Requested in Ticket ID: 1632606