Reorganize root folders

[ShishkinDmitriy]

The folder structure should be:

• plugins - contains all plug-in projects;
• features - contains all feature projects;
• releng - contains projects related to the release engineering, like
  • the project containing the parent POM;
  • the aggregator project that contains the aggregator POM which defines the modules of a build and is also the starting point of a Tycho build;
  • the target definition project that contains the target definition for the Tycho build;
• tests - contains all test plug-in/fragment projects;