Final adjustments to SLIME Trace Dialog before documentation update.

* contrib/slime-trace-dialog.el (slime-trace-dialog): simplify contrib
definition.
(slime-trace-dialog-mode): Also activate
`slime-trace-dialog-minor-mode'.
(slime-trace-dialog-minor-mode): New minor mode.
(slime-trace-dialog--menubar): New single top-level menu for
`slime-trace-dialog-minor-mode'.
(slime-trace-dialog-after-toggle-hook): New hook.
(slime-trace-dialog-toggle-trace): Run hooks.
(slime-trace-dialog-toggle-complex-trace): New interactive
function for calling from the menu.
(slime-trace-dialog--easy-menu): Delete it.
(my-menu): Delete it.