Ability to disable validation

Since decades "Wild Web Developers" is annoying users with its enforced validation (see e.g. https://github.com/eclipse-platform/eclipse.platform.ui/issues/943 - also look back to the old bugzilla issues that had been voted by many users, you can also do a web-search and find many stackoverflow and other hits from annoyed users) and no ability to disable this undesired behaviour.

Rationale

Why is it a problem? In general files should be valid, right?
But, the reality is different:

• For tests it is sometimes required to create invalid files. So e.g. if I want to test how my code reacts to incorrect JSON or XML data, I want to be able to have such invalid files in my codebase.
• "Wild Web Developers" tries to do complex validation that includes downloading of XSDs from the Internet, etc. that can fail for arbitrary technical reasons (Network issues, (temporary) Server unavailability, etc.). In such case my file can be entirely correct and the error marker for hundreds of e.g. XML files is absolutely not helpful.
• "Wild Web Developers" validation is slow. Validating every file in a very large project is a disaster.

Eclipse has a smart concept to aggregate errors and warnings.
As a result you see a project has a red cross if there are errors or not if there are no errors.
The aim of every reasonable developer is to get rid of the red crosses.
It is therefore in-acceptable to have errors that are actually false positives (as illustrated above).

Acceptance criteria

• In the preferences I want to have an option to disable/suspend all validators (maybe additionally even by type - XML, JSON, etc.).
• If that option is checked, then no validation should take place, no performance is wasted for validation and especially no validation errors are produced by upcoming builds.

The issue names preferences, validators, and upcoming builds but no files or tests. Start by tracing where validator preferences are defined and where validation runs during builds. Done means a preference can suspend all validation, avoids validation work, and produces no validation errors when enabled.