Clean build.sbt

[sebinside]

Description

After the project rework, the main build.sbt becomes somewhat cluttered. Since it seems to be possible, to split up all the information in multiple sbt files, we should do that, e.g. a dependencies.sbt like in the build project. Also, the project structure should be not documented in a sbt file, as mentioned in #123.

/*
 * A brief introduction of the sbt related folder structure:
 *       root
 *       |  build.sbt
 *       |  plugins.sbt (produced by the fetch task)
 *       |  -> api project (required to build plugins and the framework)
 *       |  -> a plugin source directory
 *       |  -> -> a plugin folder = plugin
 *       |  -> -> -> build.sbt
 *       |  -> -> -> source etc.
 *       |  -> -> another folder = another plugin
 *       |  -> -> -> build.sbt
 *       |  -> -> -> source etc.
 *       |  -> another plugin source directory (optional)
 *       |  -> gui project (build will be skipped, if missing)
 *       |  -> bootstrap launcher (for end-user deployments)
 *       |  -> build project (contains code for all sbt tasks and sbt related things)
 */