Consolidade pull-down menus

Work in progress.

* sly.el (sly-inspector-mode-map): Remove some invalid
keybindings.
(menubar-sly, menubar-sly-sldb): Removed.
(sly-menu, sly-sldb-menu, sly-inspector-menu): New menus.
(sly-sldb-easy-menu, sly-easy-menu): These variables are unneeded.
(sly-add-easy-menu, sly-sldb-add-easy-menu): Removed.

* contrib/sly-mrepl.el (sly-mrepl--last-prompt-overlay):
Resurrected.
(sly-mrepl-mode): Remove `sly-mrepl-mode-map' comint parent.
(sly-mrepl--shortcut-menu, sly-mrepl--menu): New menus.

* contrib/sly-trace-dialog.el (sly-trace-dialog--shortcut-menu)
(sly-trace-dialog--menu): New menus.
(sly-trace-dialog--menubar): Remove.