Reduce technical debt

* Reduce duplicate code
* Use more specific functional interfaces where available
* Fix some potential NPEs
* Remove some unnecessary/nonfunctional code
* Merge inventory listeners - no longer need to keep separate due to event availability
* Removed TODO items that probably won't ever be implemented. Good ideas, too drastic changes or too much work to maintain.

plugin/src/main/java/com/lishid/openinv/commands/SearchEnchantCommand.java

@@ -31,6 +31,7 @@ import org.bukkit.inventory.Inventory;
 import org.bukkit.inventory.ItemStack;
 import org.bukkit.inventory.meta.ItemMeta;
 import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
 
 /**
  * Command adding the ability to search online players' inventories for enchantments of a specific
@@ -124,8 +125,9 @@ public class SearchEnchantCommand implements TabExecutor {
         return true;
     }
 
-    private boolean containsEnchantment(Inventory inventory, Enchantment enchant, int minLevel) {
+    private boolean containsEnchantment(Inventory inventory, @Nullable Enchantment enchant, int minLevel) {
         for (ItemStack item : inventory.getContents()) {
+            //noinspection ConstantConditions // Spigot improperly annotated, should be ItemStack @NotNull []
             if (item == null || item.getType() == Material.AIR) {
                 continue;
             }