ENH: Build ParameterSerializer even if BUILD_SLICER_EXTENSION is on

Since ParameterSerializer isn't part of the slicer build, it should be
superbuild even if BUILD_SLICER_EXTENSION is on. This follows what Jsoncpp
does.
ENH: Add USE_SYSTEM_SlicerExecutionModel

Since Slicer already packs SlicerExecutionModel, one should be able to use
the system SlicerExecutionModel.
ENH: External_*.cmake projects

WIP
-- WIP -- -