Enum-Ify ISortSource since it always uses the same enum types

Reactivate JEI integration

src/main/java/appeng/integration/abstraction/JEIFacade.java

@@ -0,0 +1,20 @@
+package appeng.integration.abstraction;
+
+import com.google.common.base.Preconditions;
+
+public final class JEIFacade {
+
+    private JEIFacade() {
+    }
+
+    private static IJEI instance = new IJEI.Stub();
+
+    public static IJEI instance() {
+        return instance;
+    }
+
+    public static void setInstance(IJEI jei) {
+        instance = Preconditions.checkNotNull(jei);
+    }
+
+}