Add the getSlot method to BlockDispenseEvent. Adds BUKKIT-5142

#### The Issue:

Currently, there is no way to get which slot of a dispense/dropper was used from a `BlockDispenseEvent`. This makes it impossible for rollback plugins from determining how to reset a dispenser/dropper's inventory. 

#### Justification for this PR:

Adding the getSlot method allows plugin developers to detect which slot was used for dispensing an item. This can be used in a, for example, an antigrief/rollback plugin, which stores the slot used, and later adds the appropriate item to it.

#### PR Breakdown:

I've added a `slot` parameter to the constructor of `BlockDispenseEvent`, and added the `getSlot` method to return the specified slot.

#### Testing Results and Materials:

I've created a plugin called [SlotTester](https://www.dropbox.com/s/7i7k0urp5881bl7/SlotTester.jar). To use it, compile the CraftBukkit in the PR against my Bukkit PR listed in the Relevant PRs section. Load the plugin, then load and fire a dispenser of hopper. You will see "Slot: #" printed in the chat, where "#" is the number of the slot used, from 0 to 8. The source code can be viewed [here](https://github.com/Aaron1011/SlotTester).

#### Relevant PR(s):

B-

#### JIRA Ticket:

BUKKIT-5142 - https://bukkit.atlassian.net/browse/BUKKIT-5142