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'.

include/menu/menu.h

@@ -23,6 +23,7 @@ struct menuitem {
 	char *text;
 	char *icon_name;
 	const char *arrow;
+	char accelerator;
 	struct menu *parent;
 	struct menu *submenu;
 	bool selectable;
@@ -66,6 +67,19 @@ struct menu {
 /* For keyboard support */
 void menu_item_select_next(void);
 void menu_item_select_previous(void);
+
+/**
+ * menu_item_select_by_accelerator - selects the next menu item with
+ * a matching accelerator, starting after the current selection
+ *
+ * @accelerator a shortcut to quickly select/open an item, defined in menu.xml
+ * with an underscore in the item label before the target letter.
+ *
+ * Return: a boolean value that represents whether the newly selected item
+ * needs to be executed.
+ */
+bool menu_item_select_by_accelerator(char accelerator);
+
 void menu_submenu_enter(void);
 void menu_submenu_leave(void);
 bool menu_call_selected_actions(void);
@@ -100,7 +114,7 @@ void menu_open_root(struct menu *menu, int x, int y);
 void menu_process_cursor_motion(struct wlr_scene_node *node);
 
 /**
- *  menu_close_root- close root menu
+ *  menu_close_root - close root menu
  *
  * This function will close server.menu_current and set it to NULL.
  * Asserts that server.input_mode is set to LAB_INPUT_STATE_MENU.

src/menu/menu.c

@@ -2,6 +2,7 @@
 #define _POSIX_C_SOURCE 200809L
 #include "menu/menu.h"
 #include <assert.h>
+#include <ctype.h>
 #include <libxml/parser.h>
 #include <signal.h>
 #include <stdlib.h>
@@ -143,6 +144,16 @@ item_create(struct menu *menu, const char *text, const char *icon_name, bool sho
 	menuitem->text = xstrdup(text);
 	menuitem->arrow = show_arrow ? "›" : NULL;
 
+	const char *it = text;
+	menuitem->accelerator = tolower(*it);
+	while (*it != '\0') {
+		if (*it == '_' && *it != '\0') {
+			menuitem->accelerator = *(++it);
+			break;
+		}
+		it++;
+	}
+
 #if HAVE_LIBSFDO
 	if (rc.menu_show_icons && !string_null_or_empty(icon_name)) {
 		menuitem->icon_name = xstrdup(icon_name);
@@ -1460,6 +1471,50 @@ menu_item_select_previous(void)
 	menu_item_select(/* forward */ false);
 }
 
+bool
+menu_item_select_by_accelerator(char accelerator)
+{
+	struct menu *menu = get_selection_leaf();
+	if (!menu) {
+		return false;
+	}
+
+	bool needs_exec = true;
+	bool matched = false;
+
+	struct menuitem *selection = menu->selection.item;
+	struct wl_list *start = selection ? &selection->link : &menu->menuitems;
+	struct wl_list *current = start;
+	struct menuitem *item = NULL;
+	struct menuitem *next_selection = NULL;
+	do {
+		current = current->next;
+		item = wl_container_of(current, item, link);
+		if (!matched && item->accelerator == accelerator) {
+			/* Menuentry with a matching accelerator found */
+			next_selection = item;
+			matched = true;
+		} else if (matched && item->accelerator == accelerator) {
+			/* Another menuentry with such accelerator found,
+			   cycle selection instead of executing */
+			needs_exec = false;
+			break;
+		}
+	} while (current != start);
+
+	if (next_selection) {
+		menu_process_item_selection(next_selection);
+		if (needs_exec && next_selection->submenu) {
+			/* Submenu was opened, select the first menuitem
+			   without executing */
+			needs_exec = false;
+			menu_submenu_enter();
+		}
+	}
+	return needs_exec;
+}
+
+
 bool
 menu_call_selected_actions(void)
 {