Add callback event handling

The EventHandler class has been converted to an interface, with the
reflection specific part being moved to the new ReflectiveEventHandler
class. Callback event handlers implement the same interface through the
CallbackEventHandler class.

The event handlers are defined in the new handler package, which is not
exported by the eventbus.core module.

@@ -241,0 +277,4 @@
+	 * @since 1.2.0
+	 * @see #registerListener(Class, Consumer, boolean, int)
+	 */
+	public <E> void registerListener(Class<E> eventType, Consumer<E> callback,

@@ -241,0 +293,4 @@
+	 * @since 1.2.0
+	 * @see #registerListener(Class, Consumer, boolean, int)
+	 */
+	public <E> void registerListener(Class<E> eventType, Consumer<E> callback, int priority) {

@@ -241,0 +307,4 @@
+	 * @param priority    the priority to assign to the listener
+	 * @since 1.2.0
+	 */
+	public <E> void registerListener(Class<E> eventType, Consumer<E> callback, boolean polymorphic,

@@ -0,0 +9,4 @@
+ * @author Kai S. K. Engelbart
+ * @since 1.2.0
+ */
+public class CallbackEventHandler implements EventHandler {

@@ -0,0 +12,4 @@
+public class CallbackEventHandler implements EventHandler {
+
+	private final Class<?>			eventType;
+	private final Consumer<Object>	callback;

@@ -0,0 +20,4 @@
+	 * Constructs a callback event handler.
+	 *
+	 * @param <E>         the event type of the handler
+	 * @param eventType   the event type of the handler

@@ -0,0 +30,4 @@
+	public <E> CallbackEventHandler(Class<E> eventType, Consumer<E> callback, boolean polymorphic,
+		int priority) {
+		this.eventType		= eventType;
+		this.callback		= (Consumer<Object>) callback;

@@ -0,0 +45,4 @@
+	int getPriority();
+
+	/**
+	 * @return whether this handler is polymorphic