#21 Generalize "Mark as spam"

[Anonymous]

It may be useful to be able to mark messages matching
a filter condition as something other than "spam"
(e.g., "possible spam", "not spam", "priority mail", etc.).
All that would be needed would be the ability to define
mail categories (spam being one of them) and changing
the "spam" in "Mark as spam" to a combobox.