Some aggressive cleanup and scaffolding for UI uniformization

* sly.el (sly-popup-buffer-mode-map, sly-popup-buffer-mode): Move
up here. Don't add to `minor-mode-alist'.
(sly-define-keys): Removed.
(sly-with-popup-buffer): Only call `sly-popup-buffer-mode' if no
mode was passed.
(sly-edit-definition-cont, sly-maybe-show-xrefs-for-notes)
(sly-xrefs, sly-xref--show-results, sly-xref, sly-xref-next-line)
(sly-edit-uses, sly-xref-prev-line): Conform to new sly-xref
naming.
(sly-edit-value-callback): Don't invert `sly-popup-buffer-mode'.
(sly-apropos-mode-map): New map (empty for now).
(sly-apropos-mode): New major mode.
(sly-xref-goto): Renamed from `sly-goto-xref'
(sly-xref-show): Renamed from `sly-show-xref'
(sly-xref--show-results): Renamed from `sly-show-xrefs'
(sly-xref--show-buffer): Renamed from `sly-show-xref-buffer'
(sly-xref--show-location): Renamed from `sly-show-xref-location'
(sly-xref-mode-map, sldb-mode-map)
(sly-inspector-mode-map, sly-connection-list-mode-map)
(sly-thread-control-mode-map)
(sly-macroexpansion-minor-mode-map): Define map inline
(sldb-setup): Use `inhibit-read-only'.

* contrib/sly-compiler-notes-tree.el
(sly-compiler-notes-mode-map): Define map inline.

* contrib/sly-scratch.el (sly-scratch-mode-map): Define map
inline.

* contrib/sly-sprof.el (sly-sprof-browser-mode-map): Define map
inline.

* contrib/sly-trace-dialog.el (sly-trace-dialog-mode-map):
Scaffold the standard "q", "v", ".", "i", and "M-RET" bindings.