generalize buffer icons

generalize buffer icons

Let all buffers have an icon, and the favicon module's role is now simply
to set that value when active.  This change allows the consumers of buffer
icons to be decoupled from the favicon module, not need to break abstraction
barriers by being aware of its internal details.

buffer_favicon_change_hook renamed to buffer_icon_change_hook
current_buffer_favicon_change_hook renamed to current_buffer_icon_change_hook