skins2: improve $N text variable

skins2: improve $N text variable

Instead of hardcoding what we think the name of the current item is, make it
configurable via the --input-title-format parameter.

This change makes skins2 more consistent with qt4 that already uses this
means to set the title of the main window.

Default display will now be "artist + title" instead of just "title".
Users who would still prefer the previous behaviour can get it with
--input-title-format '$t'

This fixes a request described at http://forum.videolan.org/viewtopic.php?f=15&t=108976