(ada-parse-prj-file): Don't delete project buffer; user may want to edit it.

(ada-parse-prj-file): Don't delete project buffer; user may want to edit it.
(ada-xref-set-project-field, ada-xref-current-project-file,
ada-xref-current-project, ada-show-current-project,
ada-set-main-compile-application): New functions.
(ada-xref-get-project-field, ada-require-project-file):
Normalize use of ada-prj-default-project-file.
(ada-gdb-application, ada-get-ada-file-name, ada-make-body-gnatstub):
Normalize use of ada-require-project-file.
(ada-prj-find-prj-file): Improve doc string, comments.