Start using execute_handler for menus

This allows hiding execute_menu_cb().
Use execute_handler in apropos_menu
Use execute_handler in bookmarks_menu
Use execute_handler in history_menu

For command-line and search histories.
Use execute_handler in colorscheme_menu
Use execute_handler in commands_menu
Name commands menu related things consistently

Use "commands" in plural.
Use execute_handler in filetypes_menu
Use execute_handler in dirstack_menu
Use execute_handler in jobs_menu
Use execute_handler in volumes_menu
Use execute_handler in {find,locate,users}_menu
Use execute_handler in grep_menu
Use execute_handler in {dirhistory,trashes}_menu
Remove menus/menus.c:execute_menu_cb() function

It's not needed more.
Fix minor issues with documentation on menus
Document additional commands for more menu types
Merge branch 'use-execute-handler-in-menus'

This moves most of menu operations handling from menus.c to dedicated
files for each menu.