Add ButtonClick message handler

game/src/main/kotlin/org/apollo/game/plugin/kotlin/message/ButtonClick.kt

@@ -0,0 +1,40 @@
+package org.apollo.game.plugin.kotlin.message
+
+import org.apollo.game.message.impl.ButtonMessage
+import org.apollo.game.model.entity.Player
+import org.apollo.game.plugin.kotlin.KotlinPluginScript
+import org.apollo.game.plugin.kotlin.MessageListenable
+import org.apollo.game.plugin.kotlin.PlayerContext
+
+/**
+ * Registers a listener for [ButtonMessage]s that occur on the given [button] id.
+ *
+ * ```
+ * on(ButtonClick, button = 416) {
+ *     player.sendMessage("You click the button.")
+ * }
+ * ```
+ */
+fun KotlinPluginScript.on(
+    listenable: ButtonClick.Companion,
+    button: Int,
+    callback: ButtonClick.() -> Unit
+) {
+    on(listenable) {
+        if (this.button == button) {
+            callback()
+        }
+    }
+}
+
+class ButtonClick(override val player: Player, val button: Int) : PlayerContext {
+
+    companion object : MessageListenable<ButtonMessage, ButtonClick>() {
+        override val type = ButtonMessage::class
+
+        override fun createContext(player: Player, message: ButtonMessage): ButtonClick {
+            return ButtonClick(player, message.widgetId)
+        }
+    }
+
+}