Notification of string changes

Author: Paul Holden
MDL-66917 admin: validate uploaded plugin version.
Prevent overwriting current plugins with older versions of themselves.


http://git.moodle.org/gw?p=moodle.git;a=commit;h=536c771a3f576b1eee7d5d03d0164d1c0e067f26
http://github.com/moodle/moodle/commit/536c771a3f576b1eee7d5d03d0164d1c0e067f26

+ 3.7 en [validationmsg_pluginversiontoolow,core_plugin]