Added a showcases folder

A "showcases" folder is added to the apps directory. It should - in future - contain sources for simulations that show specific aspects of waLBerla, probably in a very appealing way. Examples are: Marble run, hour glass, fluidized bed, ... Those simulations do not benchmark or test certain parts in the traditional meaning, and also are not meant to be a tutorial. The generated output (e.g. vtk) can be used to generate a video, which could be put onto the waLBerla website, with a link to the specific showcase source code. All in all, it will be a collection of "real life" applications that can be simulated with waLBerla and will serve as a good starting base for further application developments.