menu: implement menu accelerators

Menu accelerators are one-letter mnemonics to quickly select/exec
items from the current menu. For each menu item, the accelerator is
defined as the first character of the item label, converted to
lowercase. A different accelerator can be explicitly defined in
menu.xml with the special '_' character before the target letter.

- Add a field `accelerator` to the `menuitem` struct
- Implement `menu_item_select_by_accelerator()`

Example:
The accelerator for an item with the label "e_macs" is 'm'.

src/input/keyboard.c

@@ -444,15 +444,24 @@ handle_menu_keys(struct keysyms *syms)
 			break;
 		case XKB_KEY_Return:
 		case XKB_KEY_KP_Enter:
-			menu_call_selected_actions();
+			if (!menu_call_selected_actions()) {
+				menu_submenu_enter();
+			};
 			break;
 		case XKB_KEY_Escape:
 			menu_close_root();
 			cursor_update_focus();
 			break;
-		default:
-			continue;
-		}
+		default: {
+			uint32_t accelerator = xkb_keysym_to_utf32(syms->syms[i]);
+			if (accelerator == 0) {
+				continue;
+			}
+			if (menu_item_select_by_accelerator(accelerator)) {
+				menu_call_selected_actions();
+			}
+			break;
+		}}
 		break;
 	}
 }