This change adds the following API:

* org.eclipse.core.resources.IFile.isContentRestricted()
* org.eclipse.core.resources.IFile.setContentRestricted(boolean)

The goal of this API is to allow Eclipse-based IDEs to restrict certain
platform functionality on a per-file basis, for sensitive files.

The flags are not persisted, to enable restricted handling
these flags must be set for each session.

The restrictions will apply to following functionality:

* File search
* Storing file history

Potentially more platform functionality will be restricted.

See: https://github.com/eclipse-platform/eclipse.platform/issues/2588