Add BlockMultiPlaceEvent. Adds BUKKIT-5558

Some blocks, such as beds, doors, or flowers, are actually composed of
multiple blocks when they are placed.  Currently, to detect how many
blocks are actually modified a plugin has to perform various calculations
to determine the directions of relative blocks, many of which are
difficult to perform and can easily return false positives.

This commit adds in a BlockMultiPlaceEvent, which adds in easy support for
accessing all blocks modified by the placement of a block.

By: Nate Mortensen <nate.richard.mortensen@gmail.com>

paper-api/src/main/java/org/bukkit/event/block/BlockMultiPlaceEvent.java

@@ -0,0 +1,36 @@
+package org.bukkit.event.block;
+
+import com.google.common.collect.ImmutableList;
+import org.bukkit.block.Block;
+import org.bukkit.block.BlockState;
+import org.bukkit.entity.Player;
+import org.bukkit.inventory.ItemStack;
+
+import java.util.List;
+
+/**
+ * Fired when a single block placement action of a player triggers the
+ * creation of multiple blocks(e.g. placing a bed block). The block returned
+ * by {@link #getBlockPlaced()} and its related methods is the block where
+ * the placed block would exist if the placement only affected a single
+ * block.
+ */
+public class BlockMultiPlaceEvent extends BlockPlaceEvent {
+    private final List<BlockState> states;
+
+    public BlockMultiPlaceEvent(List<BlockState> states, Block clicked, ItemStack itemInHand, Player thePlayer, boolean canBuild) {
+        super(states.get(0).getBlock(), states.get(0), clicked, itemInHand, thePlayer, canBuild);
+        this.states = ImmutableList.copyOf(states);
+    }
+
+    /**
+     * Gets a list of blockstates for all blocks which were replaced by the
+     * placement of the new blocks. Most of these blocks will just have a
+     * Material type of AIR.
+     *
+     * @return immutable list of replaced BlockStates
+     */
+    public List<BlockState> getReplacedBlockStates() {
+        return states;
+    }
+}