stbt.Region.contains: Accept a `Position` as the argument
stbt.Region.contains: Accept a `Position` as the argument

Sometimes when you've identified a selection (with stbt.match or some
other mechanism) you want to tell which row/cell it corresponds to (in a
table/grid) and it doesn't matter if the selection is slightly larger
than the cell, so you use `cell.contains(selection.region.center)`.

stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester