Support non-square aspect ratios

Removed old flag `constrain` which always locked the aspect ratio to 1:1. New flag `aspectRatio` allows setting any aspect ratio. Also fixed that `selectAll` did not trigger `crop.data`.
Also there is a regression. One can no longer make a selection going down-left or up-right when `constrain`/`aspectRatio` is active. I'm not 100% sure how I caused this but would be thankful for help on finding a fix.