Forms API, part 1: add Player->sendForm() and Form interface

There's no implementation here yet, but that can come later. This lays the ground for allowing plugins to have an integrated method to send forms, as well as a solution to the ID conflict problem.

A built in implementation should not be a concretion and it should be able to be swapped for third party implementations. This enables the possiblity to do so.

src/pocketmine/form/Form.php

@@ -0,0 +1,44 @@
+<?php
+
+/*
+ *
+ *  ____            _        _   __  __ _                  __  __ ____
+ * |  _ \ ___   ___| | _____| |_|  \/  (_)_ __   ___      |  \/  |  _ \
+ * | |_) / _ \ / __| |/ / _ \ __| |\/| | | '_ \ / _ \_____| |\/| | |_) |
+ * |  __/ (_) | (__|   <  __/ |_| |  | | | | | |  __/_____| |  | |  __/
+ * |_|   \___/ \___|_|\_\___|\__|_|  |_|_|_| |_|\___|     |_|  |_|_|
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * @author PocketMine Team
+ * @link http://www.pocketmine.net/
+ *
+ *
+*/
+
+declare(strict_types=1);
+
+namespace pocketmine\form;
+
+use pocketmine\Player;
+
+/**
+ * Form implementations must implement this interface to be able to utilize the Player form-sending mechanism.
+ * There is no restriction on custom implementations other than that they must implement this.
+ */
+interface Form extends \JsonSerializable{
+
+	/**
+	 * Handles a form response from a player.
+	 *
+	 * @param Player $player
+	 * @param mixed  $data
+	 *
+	 * @return Form|null a form which will be opened immediately (before queued forms) as a response to this form, or null if not applicable.
+	 * @throws FormValidationException
+	 */
+	public function handleResponse(Player $player, $data) : ?Form;
+}

src/pocketmine/form/FormValidationException.php

@@ -0,0 +1,28 @@
+<?php
+
+/*
+ *
+ *  ____            _        _   __  __ _                  __  __ ____
+ * |  _ \ ___   ___| | _____| |_|  \/  (_)_ __   ___      |  \/  |  _ \
+ * | |_) / _ \ / __| |/ / _ \ __| |\/| | | '_ \ / _ \_____| |\/| | |_) |
+ * |  __/ (_) | (__|   <  __/ |_| |  | | | | | |  __/_____| |  | |  __/
+ * |_|   \___/ \___|_|\_\___|\__|_|  |_|_|_| |_|\___|     |_|  |_|_|
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * @author PocketMine Team
+ * @link http://www.pocketmine.net/
+ *
+ *
+*/
+
+declare(strict_types=1);
+
+namespace pocketmine\form;
+
+class FormValidationException extends \RuntimeException{
+
+}