Better handling of accents in filenames

My media "files" are mostly Youtube videos, often with accents in their names.

By default, media_gallery tries to process the filename to make it more readable, but only taking A-Z and 0-9 into account, making accented text less readable than it used to be ("Titanic à la flûte" becoming "Titanic La Fl Te" for example).

Here's a patch that uses Unicode classes instead of ranges like [A-Z].
It shouldn't cause any issue, and the "u" regex modifier exists since PHP 4.3.5 (without any particular extension) so no worries about compatibility.