A new inTransaction() method might be a very useful shortcut.

Calling is unnecessary long right now.

/* @var \Nette\Database\Connection $db */
if ($db->getPdo()->inTransaction()) {
   $db->rollBack();
}

Why not just use it like this?

/* @var \Nette\Database\Connection $db */
if ($db->inTransaction()) {
   $db->rollBack();
}

Other shortcuts like beginTransaction(), commit() and rollback() are already implemented in Connection class.