few insecure fixes

[kordan]

I made some changes in order to drop warnings and make the code better and more uniform.
I AM NOT SURE ABOUT THE FIXES I ADDED.
It seems to me that the code works fine but I have to test it better.
Please review and merge this PR.

Among fixes:

• no more warnings (with debugger set to developer)
• customint1 and customint2 were dropped
• customint3, customint4 and customint5 were renamed to customint1, customint2 and customint3
• added notificationeabc:config capability to override public function can_hide_show_instance($instance)
• overridden public function can_hide_show_instance($instance)
• icons in Administration > Course administration > Users > Enrolment methods are now homogeneous with the other enrolment plugin
• few (not all) variables renamed to corresponding name in english
• strings key made homogeneous
• some lang strings slightly changed
• increased version number

[e-abc]

Hi Kordan,
Awesome work.
I only ask you to modify the string location in the settings.php file as well and I accept the change
Thanks

[kordan]

Do not blindly trust me, please!
Test your code before. I have never tested it seriously.
About the string location, I don't know what is your project and I can not change the code according to your needs. You are the maintainer. I am only a user PROPOSING an improvement.