Add preference for disabling indexing of specific files

[trancexpress]

We have some sources, which we want to exclude from the Java indexing. We wish to add a preference for this and mark the files with a session property. If both the preference is set and the session property is present at a file, the file should be excluded from indexing.

See also:

• Add preference for disabling history on specific files eclipse-platform/eclipse.platform#2588
• Provide options for disabling file search on some files eclipse-platform/eclipse.platform.ui#3815