Add menu configuration submenu (Menu  Options)

It add a new sub-menu to the configuration menu, named
'menu options'.

With it, users can now choose between different
blackbox-menu files, without manually editing
blackboxrc

Choices given are:
- the file specified in the blackboxrc
- the default user menu (~/.blackbox/blackbox-menu in Debian),
- the default system menu.

A new corresponding item has been added to blackboxrc,
session.menuType, which can take any of the following values:
Defined, User or System.