Fire BlockPlaceEvent for Half Slab Completion - Fixes BUKKIT-5390

The Issue:

A diff was either missed or new code was added in 1.7 to handle completing double slab placement when clicking adjacent blocks, resulting in a block place action not firing the block place event.

Justification for this PR:

Fixes a bug that the BlockPlaceEvent will not fire when a user completes a half slab by clicking an adjacent block. This breaks protection plugins ability to properly restrict placement, as well as misses logging for logging plugins.


PR Breakdown:

Adds Missing processBlockPlace call to ItemStep for when a user completes a slab by clicking an adjacent block.

JIRA Ticket:

https://bukkit.atlassian.net/browse/BUKKIT-5390