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