New generic print_menu() API function

Prior to this, the code to generate menus was duplicated in several HTML
API functions:

• print_manage_menu()
• print_account_menu
• print_doc_menu
• print_summary_menu

This commit moves the logic in a new print_menu() function, called by
the above.

Another new function plugin_menu_items() takes care of retrieving
plugin-defined menu items for a given event.