[Users] theme icons

[Ian Nartowicz]

The ham_btn icon only appears for spam items.  Look in your spam folder and
it should show up on the toolbar.  It also appears in the palette of icons
that you can pick for toolbar items.

Your rot13 button is a custom action.  It does not have an icon in the
default theme and you can't define one yourself.  You can pick an icon from
the palette for custom actions, or Claws will pick one for you.  I think it
just picks the same icon as the first button currently on the toolbar, not
very helpful.

It would be nice to be able to define additional icons for the palette, but
you can't.  However, it is worth noting that there are at least two icons in
the palette that are never used in any current or recent version of Claws,
so you could use those as custom icons.  These are the notice_note and
notice_error icons, which used to appear on dialogs but are no longer used.
 The notice_warn icon is used, so don't mess with that.