Seconds to hours

Currently duration in smartplaylists is limited to 99:59 in the UI.  It's stored as seconds, so this isn't long enough for most movies.

This changes the INPUT_TYPE_SECONDS mode of the numeric dialog to input HH:MM:SS.  It's not used anywhere other than for time fields in smartplaylists, so should be quite safe, even for Gotham.

Further, it alters the string validator IsTime() to support both HH:MM:SS and MM:SS (and SS).  Note there's no validation on MM and SS being less than 60 in the former two (there wasn't before either) though this could be added if we like?

@Montellese please review.
@t-nelson flagged for Gotham.X