New plugin: source code validator

[Wiktor Walc]

Perhaps some day in the future it would make sense to make some kind of source code validator plugin to check the correctness of the HTML code modified by user.

There are cases like #13306 where it's even hard to notice that the source code was broken as a result of manual changes in the code. Some tool that could validate the HTML content before the mode is switched back to wysiwyg could help in dealing with mistakes and prevent from creating broken content.

Of course this is a complex task, especially if we consider notifying user in some user-friendly way where the content is possible broken. In case of unclosed tags it's even harder.