Add property to determine if any rows are selected

[NikxDa]

Hey folks,

while building a DnD table, I want to highlight the rows that are selected by fading those away that aren't. For that, each row needs to know if any other row is selected. I can use selectedFlatRows.length and pass that into the row object, but it's quite tricky to get it into a cell renderer, e.g. to hide some buttons when a row is selected.

There is already a property row.isSomeSelected. I propose adding a second property, row.isAnySelected.