Notification of string changes

Author: sam marshall
MDL-41669 Restore: Progress bar needs to include more tasks
1. Changes progress bar code to allow headings for progress bar (so users have
some clue what's going on if a page has more than one progress bar).
2. Changes restore code so that a progress bar can display during pre-checks if
they take longer than 5 seconds.
3. Changes pre-check and restore code so that, in various points where the system
can take a long time within an individual step, intederminate progress is
indicated and it won't time out.


http://git.moodle.org/gw?p=moodle.git;a=commit;h=4fb31ae6cd3fee2d604b3c6c3a6854a0cd28c46e
http://github.com/moodle/moodle/commit/4fb31ae6cd3fee2d604b3c6c3a6854a0cd28c46e

+ 2.6 en [preparingui,core_backup]
+ 2.6 en [preparingdata,core_backup]