Add 'CMakeLists.txt.user' to .gitignore

This is the file user by QtCreator to store his project specific info.
See https://bugreports.qt-project.org/browse/QTCREATORBUG-733
Change project stack prefix to "SB_"

Change stack property prefix to "SB_" to match the "_sb_" prefix used
for private functions.
Change "first pass" property prefix to "SB_"

Similarly to commit 58f615760, the prefix is changed to match the
"_sb_" prefix used for private functions.
Remove need for 'superbuild_include_once' macro
Simplify logic displaying external project include state and type
Remove duplicate argument in external project arguments
Add convenience module ArtichokeCheckVariable

This will avoid to have duplicated code for check_variable helper function.
Keep source of Lib project used for testing in one folder

We will make maintenance and addition of new test case easier.
Simplify test of USE_SYSTEM variables

There is no need to pass them to the inner project to check their value.
Calling 'check_variable' after invoking 'superbuild_include_dependencies'
is sufficient.
Refactor test organization in Lib/CMakeLists.txt
Change superbuild_include_dependencies API to address issue #1

The new signature is:

#!
#! superbuild_include_dependencies(<project_name>
#!     [PROJECT_VAR <project_var>]
#!     [EP_ARGS_VAR <external_project_args_var>]
#!     [DEPENDENCIES_VAR <dependencies_var>]
#!     [USE_SYSTEM_VAR <use_system_var>]
#!    )
#!