There is now a clear distinction between:
 - userSettings -> i.e. ~/.config/NA-MIC/Slicer.ini
 - revisionUserSettings -> i.e. ~/.config/NA-MIC/Slicer-<revision>.ini

The following slicer settings are now revision specific:
 - Extensions/ManagerEnabled
 - Extensions/ServerUrl
 - Extensions/InstallPath
 - Modules/AdditonalPath
 - Modules/IgnoreModules

Additionally, the launcher version has also been updated to be able to
consider settings associated with a user, application and revision. This is
captured by the new parameter APPLICATION_REVISION, ORGANIZATION_DOMAIN,
ORGANIZATION_NAME and USER_ADDITIONAL_SETTINGS_FILEBASENAME passed to the CMake
function "ctkAppLauncherConfigure".

qSlicerCoreApplication API has also been extended and now includes methods
like:
  - QSettings* userSettings()  [settings() method is deprecated and kept for backward compatibility]
  - QSettings* revisionUserSettings()
  - QString launcherRevisionUserSettingsFilePath()
  - QString slicerUserSettingsFilePath()
  - QString slicerRevisionUserSettingsFilePath()

In addition to the settings, the extensions install path is also revision
specific, it means that it's now possible to have multiple version of slicer
running with different set of extensions.

Fixes #2791, #2143, #2720, #2533


Since the version of the launcher is updated, launcher will return EXIT_FAILURE
also when no application is been set in the launcher settings.

Fixes #2767

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21482 3bd1e089-480b-0410-8dfb-8563597acbee