David Edmundson authored Sep 10, 2021 and Vlad Zahorodnii committed Sep 10, 2021

In a recent refactor a guard seemingly in the wrong place on
SeatInterface was moved into AbstractDataSource, as typically that's
where we guard.

However it turns out the original code was correct, we want to send
cancel for all clipboard cancels, but for D&D it's version dependent.

This class introduces a new method to astract that from seat.