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