Checked in improved `mail-sources' :type

[Per Abrahamsen <abraham@dina.kvl.dk>]

Please test that 

        M-x customize-option RET mail-sources RET

looks reasonable with the latest CVS.  It does have a rather complex
and long type, so I can easily have made an error.

PS:  Someone wrote (in the old :type):

  ;; This specification should be tidied up, particularly to avoid
  ;; constant items appearing.  (Perhaps there's scope for improvment
  ;; in the widget code.)

The trick is to use 

        :format ""

to make invisible entries.  Also,

        checklist :greedy t

is a great way to present something that is essentially an property
list with a fixed set of properties (if we want to allow arbitrary
properties, the `plist' widget is even better.  Hmm...  Maybe using
`plist' would be better in any case, it would make the customization
type more robust over unknown properties.

The command `M-x widget-browse RET' is good for examining the
properties of the widgets you use.  It is particularly nice for
inspecting the :format value.