Add _selectAt(grid, index)

[mvysny]

There's GridKt.select() but it requires a bean to be passed in. A tester not familiar with the code base will have hard time figuring out how to retrieve the bean instance. We should both:

1. Document to use GridKt._get() to get the bean at the index, then assert that it's the right bean, then pass it to GridKt._select()
2. Since we already have GridKt._clickItem(grid, index), let's add _selectAt(grid, index) too

[mvysny]

Implemented as _selectRow(). Since Karibu-Testing 1.3.18.