Ladders can be placed on the tops and bottoms of blocks

If clicking on the top or bottom face of a block next to a wall or on another invalid location for a ladder, an invisible ladder block will be placed. This is very much unexpected behaviour.

The invisible ladder can be broken the same as a normal ladder, but has a full (1x1x1) bounding box, and will drop if any adjacent block is removed.

Steps to reproduce:

• Stand next to a wall, with space between yourself and the wall to place a block on the ground. (screenshot, where my crosshairs are pointing)
• Click on the block on the ground next to the wall. An invisible ladder will be placed.