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

PHPBB3-12847

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;
+	}
+}