[GEP-26] Rebuilding big workspaces can take very long

I found two problems:

1. The PPBuildJob had no rule attached to it which means that the
   scheduler was allowed to run several such jobs in parallel. This
   commit fixes that problem by adding the workspace root as a rule
   effectively forcing all scheduled PPBuldJobs to execute in
   sequence.

2. The PPModuleMetadataBuilder scheduled a PPBuildJob in order to
   rebuild all module projects. This was the main culprit for this
   issue since this resulted in a cascade of such builds being
   scheduled when a large number of project imports where made
   as one batch. The remedy was to insert a delay so that all
   requests made for such builds during the wait results in one
   job.
[GEP-558] NPE when using remote file system

This commit will ensure that only projects that are accessible using
standard file I/O are considered accessible xtext projects.