BUG: Properly serialize python paths when installing extension. Fixes #3772

Following r23036 environment variable specifying list of path were
handled differently. This commit ensures that python paths associated
with extension are also serialized into the settings by following the
new convention introduced in r23036.

For example, before this commit, after installing an extension named
"Foo", the settings contained:

// ----------------------
[Modules]
AdditionalPaths=/home/jchris/.config/NA-MIC/Extensions-23473/Foo/lib/Slicer-4.3/qt-scripted-modules

[...]

[EnvironmentVariables]
PYTHONPATH=/home/jchris/.config/NA-MIC/Extensions-23473/Foo/lib/Slicer-4.3/qt-scripted-modules
// ----------------------

whereas the following is now written:

// ----------------------
[Modules]
AdditionalPaths=/home/jchris/.config/NA-MIC/Extensions-23473/Foo/lib/Slicer-4.3/qt-scripted-modules

[...]

[PYTHONPATH]
1\path=/home/jchris/.config/NA-MIC/Extensions-23473/Foo/lib/Slicer-4.3/qt-scripted-modules
size=1
// ----------------------

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23482 3bd1e089-480b-0410-8dfb-8563597acbee