#1029: Trial changing a small number of inner enums to classes/interfaces to better support custom values

This PR is a subset of the enum PR #660 and is designed as a low impact
trial run of the design and backwards compatibility to inform subsequent
development.

Additional plugin compatibility features may be available by setting
`settings.compatibility.enum-compatibility-mode` to `true` in
`bukkit.yml`.

By: DerFrZocker <derrieple@gmail.com>

paper-api/src/main/javadoc/overview.html

@@ -31,5 +31,10 @@
     <li>Implementation classes. Concrete implementation classes packaged with
     Bukkit (eg those beginning with Simple) are not API. You should access them
     via their interfaces instead.</li>
+    <li>The "enumness" of enums which implement {@link org.bukkit.Keyed}.
+    Such enums are liable to conversion to regular classes and therefore
+    usages deriving from their status as an enum (including, but not limited
+    to, use in switch statements and classes such as {@link java.util.EnumSet})
+    should be avoided.</li>
     </ul>
 </body>