Add preference for dragged selection to respect snap to grid setting

I find it very unintuitive that dragged selections don’t respect the snap to grid setting.
This is inconsistent with the behaviour of other tools in Aseprite and the behaviour of other programs I’ve used.

It’d be nice if a preference was added to make the selection tool respect the snap to grid setting (and Alt key effect could be reversed in this case to modelessly disable snapping when snap to grid is enabled).