Because multiple clients may use the same style list, some mechanism is
needed to know when a style list should be destroyed. Calling AdjustUsage with addUser = TRUE increments the reference
counter on the style list, so that it won't be destroyed by another user.
If addUser = FALSE, the reference count is decremented. The
style list is not automatically destroyed if the reference count
goes to 0.
A style list should only be destroyed when
wxStyleList::IsUsed (section 
) returns FALSE.