Snaps dragged item to end of collection if dragged past bounds

[calvin-godfrey]

This is most applicable for grids and corresponds with issue #118.

If the number of items in a grid is not a perfect multiple of the column/row count, then there can be empty slots at the end of a grid. For example, with 8 items and 3 rows, the layout is:

0 1 2
3 4 5
6 7

In this situation, if the user drags 5 straight down, their intent is likely to move it to the end of the grid, but because the center of 5 is never contained with 7, the code to swap them never runs. Alternately, if the margin between items is big enough, it's not that difficult to drag 4 diagonally between 5 and 7 and have the same issue.

Because there's no way to get the full number of items in the collection without introducing a breaking API change, my code uses state.layoutInfo.visibleItemsInfo.maxOf { it.index } instead to get the highest currently rendered item. This is safe because if the current view doesn't include the end of the grid, then there's no empty spots and no behavior change.

This is my first contribution to another open source library, so please let me know if there's anything else you want me to do!

[calvin-godfrey]

Let me know what you think @Calvin-LL