Support Unicode expression modifier when validating H5P title attribute

[paulholden]

See the following line:

h5p-php-library/h5p.classes.php

Line 688 in 4237a6c

'title' => '/^.{1,255}$/',

As reported at https://moodle.atlassian.net/browse/MDL-87358 it means that content containing Unicode characters is not correctly validated