macosx: Fix year · duration strings in media item supplementary views, have have fallbacks for items with no set year

Currently, we present a bit of a borked string in media items' supplementary view when the media item has no set year (e.g. '0 · 00:34').

This MR fixes this by:

A) Trying instead to use the last modification date of the media item's first file if possible B) Falling back to a simple duration string if the above is not possible

With fix:

Bug in master:

Signed-off-by: Claudio Cambra developer@claudiocambra.com