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.