Notification of string changes

Author: Jun Pataleta
MDL-86998 message: Fix accessibility issues in notification preferences
* Fix missing column heading for the preference column
* Fix incorrect label for the toggle controls
* Make the preference name a table heading with a row scope so that the
toggle controls will be programmatically associated to it.
* Apply a col scope for the processor column headings to
programmatically associate the toggle controls to their respective
columns.


http://git.moodle.org/gw?p=moodle.git;a=commit;h=a637333419695e875fd14699f65eade0d568679d
http://github.com/moodle/moodle/commit/a637333419695e875fd14699f65eade0d568679d

+ 4.5 en [preference,core_message]