[ticket/12847] Allow the extensions to say if they can be enabled

PHPBB3-12847

tests/extension/ext/vendor3/foo/composer.json

@@ -0,0 +1,23 @@
+{
+	"name": "vendor3/foo",
+	"type": "phpbb-extension",
+	"description": "An example/sample extension to be used for testing purposes in phpBB Development.",
+	"version": "1.0.0",
+	"time": "2012-02-15 01:01:01",
+	"license": "GPL-2.0",
+	"authors": [{
+			"name": "John Smith",
+			"email": "email@phpbb.com",
+			"homepage": "http://phpbb.com",
+			"role": "N/A"
+		}],
+	"require": {
+		"php": ">=5.3"
+	},
+	"extra": {
+		"display-name": "phpBB Bar Extension",
+		"soft-require": {
+			"phpbb/phpbb": "3.1.*@dev"
+		}
+	}
+}

tests/extension/ext/vendor3/foo/ext.php

@@ -0,0 +1,20 @@
+<?php
+
+namespace vendor3\foo;
+
+class ext extends \phpbb\extension\base
+{
+	static public $enabled;
+
+	public function enable_step($old_state)
+	{
+		self::$enabled = true;
+
+		return self::$enabled;
+	}
+
+	public function is_enableable()
+	{
+		return false;
+	}
+}