External project specification have been added to SuperBuild.cmake

Running:
  1) "cmake ../Slicer4" => Build using SuperBuild
  2) "cmake -DSlicer_SUPERBUILD:BOOL=OFF ../Slicer4" => Build without Superbuild

In case (2), all external dependencies (VTK, ITK, ...) will have
to be specified manually.

Layout of the CMakeLists.txt has been reviewed to improve readability

Factored out cmake code and use macro SlicerMacroEmptyExternalProject
in VTK, ITK and CTK external project description.

${ep_common_args} has been introduced to contain option common
to all external project.

launcher.tcl.in has been updated to properly behave when KWWidgets_*_DIR
are not specified.

CTestCustom files has been merged into one file now located
in CMake subdirectory

SlicerFindQt.cmake has been removed, Qt discovery is now handled
directly in both CMakeLists.txt and SuperBuild.cmake

{Subversion_SVN, GIT}_EXECUTABLE are now discovered at superbuild
configure time. In case of error a message will be immediately thrown
in case of problem.

Dashboard template and driver script have been updated to reflect new
directory layout.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@15174 3bd1e089-480b-0410-8dfb-8563597acbee