genlist: (Mostly) fix item index odd/even styles

genlist: (Mostly) fix item index odd/even styles

This fixes the internal item order index.

Note that groups don't reset the odd/even styles. The
original code wasn't very clear on the intent (setting
to 0 in one case, not increasing the counter in another,
but that was not consistent all over the place). I believe
resetting the odd/even styles at a group boundary would
look great, but this might be for another patch :)

This amends part of another commit, but keeps its feature:
 b40a6eb85bf44a genlist: implement list position signals.

See T3086

PS: I've discovered more odd/even issues with the
    fileselector in particular. Still working on it...

@fix