multimedia: url friendly file names

[stefanorosanelli]

having file names with spaces and diacritics may be problematic: bad urls if published or used in scripts

when uploading files: change file names on file system forcing "url friendly" file names with same algorithm used for object unique names (nickname).

[didoda]

b172894 I released a patch that forces "friendly file name" when uploading a file.
Still todo: saving original name. Adding a new field to 'streams' table? using 'contents.subject' field [still unused]?

[didoda]

fcad3cc title (if not indicated) is the original file name ($data['original_name']).
If we add 'original_name' field to stream table, original file name should already be saved (I didn't add myslef, because we are still thinking about a solution on 'where save original file name?')

[didoda]

6cc59f6 added field original_name to streams table