Belt printer: bring back the raw G-code view in the canvas view menu

The "Show raw G-code (belt only)" toggle, retired in #16236, returns as
an item of the Preview canvas view menu (with its B shortcut), and only
there: no legend checkbox.  Unlit, the preview shows the designed,
upright view; lit, the raw machine-frame G-code, which is what to look
at when checking the machine frame transforms.  The toggle is view
only; exported G-code is the same either way.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

src/slic3r/GUI/Shortcuts.cpp

@@ -149,6 +149,7 @@ constexpr std::array<ShortcutInfo, size_t(Shortcut::Count)> shortcut_table = {{
     SHORTCUT(ShowWireframe,     "show_wireframe",     L("Show/Hide wireframe"),                                  CANVAS,  { WXK_RETURN, CTRL_SHIFT }),
     SHORTCUT(ToggleGcodeWindow,   "toggle_gcode_window",   L("On/Off G-code window"),                            PREVIEW, { 'C' }),
     SHORTCUT(ToggleOneLayerMode,  "toggle_one_layer_mode", L("On/Off one layer mode of the vertical slider"),    PREVIEW, { 'L' }),
+    SHORTCUT(ToggleBeltRawGcode,  "toggle_belt_raw_gcode", L("Show raw G-code (belt only)"),                     PREVIEW, { 'B' }),
 
     // Application
     SHORTCUT(Preferences,       "preferences",        L("Preferences"),                                          GLOBAL, PREFERENCES_CHORD),