Invert Y axis option

Currently the tiling y-axis starts with the height at 0 at the top and incrementing as it goes down the map. I think it would a be a nice feature, but relatively simple task, to add some kind of checkbox option to invert the Y axis so that (0,0) starts at the bottom and then (0, 1) is the tile above.

Thanks!

The issue does not name a file or test. Start by tracing how Tiled stores and displays tile-map Y coordinates, then identify the editor setting entry point; done means an option makes (0,0) bottom-origin and (0,1) the tile above without breaking existing maps.

Written by the indexing model from the issue text.