(ada-search-directories): Take into account ADA_INCLUDE_PATH for better

(ada-search-directories): Take into account ADA_INCLUDE_PATH for better
compatibility with GNAT.
(ada-contextual-menu): Menu defined through `easy-menu-define' instead.  Various
adjustments to the indentation engine (handling of subtypes, begin blocks,
etc.).
(ada-create-menu): Major rewrite of the handling of menus to use
`easy-menu-define' for cleaner code and better compatibility with XEmacs.  All
menus that were previously in ada-xref.el and ada-prj.el are now defined in this
package, which makes it easier to edit menus.
(ada-narrow-to-defun): Add support for `narrow-to-region'.  No longer
explicitely load ada-xref.el and ada-prj.el.  Use autoload statements instead.