Fix: enable Selection#selectAll method to work with non-store-backed grid

`dgrid/Selection`'s `selectAll` method calls `List#row` (for reasons I am not aware of - hopefully unimportant, or my fix may not be appropriate) and passes it an object, which causes the `row` method to assume the grid has a store. This causes the `selectAll` method to throw an error when used with a grid that does not use a store.