Introduce app info xml parser including basic unit test - necessary for ...

...#10777
I understand that this is intended to parse only the application info XML files and should therefore be safe. However, for the sake of secure coding practises I'd appreciate it if we would disable the entity loader here. See https://github.com/owncloud/core/blob/985f8c3f69a5dd940271481ba18eea784893406e/lib/private/db/mdb2schemareader.php#L47-49 for an example.