Allow styles to be layered over a base style. The list of styles to
apply is layered over the base style in order, such that if the layer
does not differ from the base it is not used. The order that these
styles are applied in is, from first to last:
msglist_default
msglist_unread
msglist_read (exclusive with unread, so technically the same level)
msglist_flagged
msglist_deleted
msglist_marked
So, msglist_marked style dominates.
This fixes an issue where the msglist_deleted style was not being applied.
f9bba3d17d