STYLE: Facilitate integration with python IDE

This commit introduces slicer/kits.py and rename slicer/init.py.in
to slicer/init,py. This will facilitate integration with IDE like
PyCharm when simply adding the "Slicer/Base/Python" folder as dependent
project for scripted module.

Ideally, on platform supporting it, python script shouldn't be copied
into the build directory. Instead they should be symlinked similarly to
what is done with the option "develop" of setup.py or the option "--editable"
of pip.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24553 3bd1e089-480b-0410-8dfb-8563597acbee