Ctrl+A and Data Sort took ages to broadcast ALL cells

... now that also empty cells are to be broadcasted.

Set dirty and broadcast only the effective data range as determined by Sort.

This is more a workaround, a cleaner solution would be to refactor the
SetDirty() algorithm to iterate only through broadcasters and use
AreaBroadcast() to notify area listeners.

However, this can also be easily backported to 4-3.

Change-Id: I6d68ca0088cec6a8328a3e93364ac928ef69babe