
  extensions  (toolbars et al.)

  (icon *i)il->theme->areas anchors a list of all extensions for icon-list il.
  each of these "icon" entries describes a single extension.
  user1 of each of these "icon" entries contains a pointer to a theme_area
  struct that contains function pointers, a shared object-handle, and space
  for extra user-data.
  each of these "icon" entries is likely to have one or more sub-entries of
  the "icon_el" type that describe the rectangle-list of the extension.
  the user1 of each of these "icon_el" entries is not used by evidence and
  may be used by the extensions to store pointers particular to the given
  rectangle.  the nature of the data pointed to may vary from extension to
  extension.

  icon
   +- user1{theme_area *}                   theme_area object
   |   +- user1,user2,user3,user4           extension-specific pointers
   |   +- lib                               library-handle
   |   +- ext_upd, ext_dst, ext_evt         functions in extension
   +- user2{void *}                         extension-specific pointer
   +- els{icon_el *}                        rectangle list of extension
   |   +- user1{file_info *}                extension-specific pointer
   |   +- xo,yo,w,h{int}                    position and area of rectangle
   |   +- ...
   |   +- next{icon_el *}                   next rectangle
   +- next{icon *}                          next extension
