show row number

[pfitzseb]

Not sure if this is what #34 wants -- with this implementation the row number changes when filtering/sorting:

[nilshg]

As the person who opened #34 two observations:

(1) I think ideally the row number would be a fixed column with different style like the column names

(2) From this follows that I also think it should be unchanged, just a very basic indicator of "how far in the current view have I scrolled down"?

(2) is potentially more controversial though, as there are probably valid use cases for both sides of the argument. I suppose my suggestion is more Excel-like, and people would be free to just add a column 1:nrow(df) to their data if they wanted a display with fixed row numbering that is persistent across re-orderings.