Fixes subtitles detection from within folders with non-ASCII names when the local encoding is not UTF-8 (closes #536)

The download specific functions should maybe be exported (i don't know if
other modules would need to download files to disk)

zorglub: could you have a look ? this solution doesn't seem right to
me because we "loose" some of the calls to the interaction system if
they're sent at the wrong moment.

* Reduce vlc_object_find calls

modules/gui/wxwidgets/dialogs/updatevlc.cpp : clean up