TOLTECS: Hide/show arrows in original save/load dialogs

The arrows are only visible when there are savegames to scroll to
in that direction.
TOLTECS: Scroll original save/load dialog to bottom on opening

This doesn't behave quite like the original - it seems the original
does not take the fake last entry into account when saving? - but I
think it's actually a bit more consistent this way.
TOLTECS: When a new dialog opens, highlight active item (if any)

We do this by acting as if the mouse just moved to its current
position. I first noticed this on the original save/load dialogs.