Give context for strings used in menus

Makes it easier to see which hot-keys might conflict with each
other, and allow some variation in translation in the future.
diff --git a/vncviewer/Viewport.cxx b/vncviewer/Viewport.cxx
index ee74e9d..a7881bc 100644
--- a/vncviewer/Viewport.cxx
+++ b/vncviewer/Viewport.cxx
@@ -787,46 +787,49 @@
 {
   contextMenu->clear();
 
-  fltk_menu_add(contextMenu, _("E&xit viewer"), 0, NULL,
-                (void*)ID_EXIT, FL_MENU_DIVIDER);
+  fltk_menu_add(contextMenu, p_("ContextMenu|", "E&xit viewer"),
+                0, NULL, (void*)ID_EXIT, FL_MENU_DIVIDER);
 
-  fltk_menu_add(contextMenu, _("&Full screen"), 0, NULL, (void*)ID_FULLSCREEN,
+  fltk_menu_add(contextMenu, p_("ContextMenu|", "&Full screen"),
+                0, NULL, (void*)ID_FULLSCREEN,
                 FL_MENU_TOGGLE | (window()->fullscreen_active()?FL_MENU_VALUE:0));
-  fltk_menu_add(contextMenu, _("Minimi&ze"), 0, NULL,
-                (void*)ID_MINIMIZE, 0);
-  fltk_menu_add(contextMenu, _("Resize &window to session"), 0, NULL,
-                (void*)ID_RESIZE,
+  fltk_menu_add(contextMenu, p_("ContextMenu|", "Minimi&ze"),
+                0, NULL, (void*)ID_MINIMIZE, 0);
+  fltk_menu_add(contextMenu, p_("ContextMenu|", "Resize &window to session"),
+                0, NULL, (void*)ID_RESIZE,
                 (window()->fullscreen_active()?FL_MENU_INACTIVE:0) |
                 FL_MENU_DIVIDER);
 
-  fltk_menu_add(contextMenu, _("&Ctrl"), 0, NULL, (void*)ID_CTRL,
+  fltk_menu_add(contextMenu, p_("ContextMenu|", "&Ctrl"),
+                0, NULL, (void*)ID_CTRL,
                 FL_MENU_TOGGLE | (menuCtrlKey?FL_MENU_VALUE:0));
-  fltk_menu_add(contextMenu, _("&Alt"), 0, NULL, (void*)ID_ALT,
+  fltk_menu_add(contextMenu, p_("ContextMenu|", "&Alt"),
+                0, NULL, (void*)ID_ALT,
                 FL_MENU_TOGGLE | (menuAltKey?FL_MENU_VALUE:0));
 
   if (menuKeySym) {
     char sendMenuKey[64];
-    snprintf(sendMenuKey, 64, _("Send %s"), (const char *)menuKey);
+    snprintf(sendMenuKey, 64, p_("ContextMenu|", "Send %s"), (const char *)menuKey);
     fltk_menu_add(contextMenu, sendMenuKey, 0, NULL, (void*)ID_MENUKEY, 0);
     fltk_menu_add(contextMenu, "Secret shortcut menu key", menuKeyCode, NULL,
                   (void*)ID_MENUKEY, FL_MENU_INVISIBLE);
   }
 
-  fltk_menu_add(contextMenu, _("Send Ctrl-Alt-&Del"), 0, NULL,
-                (void*)ID_CTRLALTDEL, FL_MENU_DIVIDER);
+  fltk_menu_add(contextMenu, p_("ContextMenu|", "Send Ctrl-Alt-&Del"),
+                0, NULL, (void*)ID_CTRLALTDEL, FL_MENU_DIVIDER);
 
-  fltk_menu_add(contextMenu, _("&Refresh screen"), 0, NULL,
-                (void*)ID_REFRESH, FL_MENU_DIVIDER);
+  fltk_menu_add(contextMenu, p_("ContextMenu|", "&Refresh screen"),
+                0, NULL, (void*)ID_REFRESH, FL_MENU_DIVIDER);
 
-  fltk_menu_add(contextMenu, _("&Options..."), 0, NULL,
-                (void*)ID_OPTIONS, 0);
-  fltk_menu_add(contextMenu, _("Connection &info..."), 0, NULL,
-                (void*)ID_INFO, 0);
-  fltk_menu_add(contextMenu, _("About &TigerVNC viewer..."), 0, NULL,
-                (void*)ID_ABOUT, FL_MENU_DIVIDER);
+  fltk_menu_add(contextMenu, p_("ContextMenu|", "&Options..."),
+                0, NULL, (void*)ID_OPTIONS, 0);
+  fltk_menu_add(contextMenu, p_("ContextMenu|", "Connection &info..."),
+                0, NULL, (void*)ID_INFO, 0);
+  fltk_menu_add(contextMenu, p_("ContextMenu|", "About &TigerVNC viewer..."),
+                0, NULL, (void*)ID_ABOUT, FL_MENU_DIVIDER);
 
-  fltk_menu_add(contextMenu, _("Dismiss &menu"), 0, NULL,
-                (void*)ID_DISMISS, 0);
+  fltk_menu_add(contextMenu, p_("ContextMenu|", "Dismiss &menu"),
+                0, NULL, (void*)ID_DISMISS, 0);
 }