Initialize repository

The structure of the application is intented for users to be able to
easily create a slicer-based custom application with little renaming while
keeping a source repository clean.

The first time the project is configured, it will add
External_YOURPROJECTNAME and External_YOURPROJECTNAMELibs to the CMake
source directory. These file are enough to compile the basic Slicer but can
be tailored to fit the application's need (for example another library).