ENH: Refactor setting of external project launcher settings.

This commit moves the definition of launcher setting into each external
projects.

The different type of setting (listed below) are passed to the the main
project (Slicer) using labels.

Type of settings and associated labels:

  Build tree:

    * LIBRARY_PATHS_LAUNCHER_BUILD
    * PATHS_LAUNCHER_BUILD
    * PYTHONPATH_LAUNCHER_BUILD
    * ENVVARS_LAUNCHER_BUILD

  Install tree:

    * LIBRARY_PATHS_LAUNCHER_INSTALLED
    * PYTHONPATH_LAUNCHER_INSTALLED
    * PATHS_LAUNCHER_INSTALLED
    * ENVVARS_LAUNCHER_INSTALLED

This patch has been tested on factory-win7, factory-south-ubuntu and
factory. The SlicerLauncherSetting.ini and SlicerLauncherSettingToInstall.ini
file have been compared with / without this patch and the file are
similar. The few differences are explained below:
* nonexistent path are not configured anymore
* on unix, following r23613, a new version of incrTcl is used and path
have been updated to reflect this. Since there are to version of incrTcl
(4.0.0 and 4.0.1), the path to "4.0.1" are considered.