Make possible to distinguish between 'loading' and 'no-data-returned'

[mbonaci]

I used spinning wheel as NoDataComponent and it works well, but the issue is that it shows whenever there is no data, even when the back end returns empty array or search doesn't find any docs. So we would need to have both NoDataComponent and LoadingComponent. FYI I can implement this (I need it right away). If I missed something here please provide feedback. Thx.

[otisg]

I've also observed situations where sematable shows "no data" sort of message, but then a few seconds later the actual data loads. So +1 for a PR.