simplify album-adding by marking itime as album

Since both albums and items have an itime field, I'm putting itime into
ALBUM_KEYS_ITEM. This simplifies a lot of handling code. It also will make the
item and album itimes synchronize at some points, which is probably fine -- I
can't see a major use case for maintaining separate added dates for an album
and its tracks.
itime (#264): name change

I changed the "itime" field to "added" and the %format function to %time.