[WinRT] Toggle selection of a list item on long press

As you probably know, the standard way to select or deselect an item in a WinRT list control is to slide it up or down a little. Although I rather like this gesture, it’s not very intuitive for users unfamiliar with Modern UI. And it gets even more confusing, because my previous statement wasn’t perfectly accurate: in fact, you have to slide the item perpendicularly to the panning direction. In a GridView, which (by default) pans horizontally, that means up or down; but in a ListView, which pans vertically, you have to slide the item left or right.