Add option allowing to reduce number of python copy/compilation targets

By globally defining the variable CTK_COMPILE_PYTHON_SCRIPTS_GLOBAL_TARGET_NAME to a
non-empty string or by specifying the macro option 'GLOBAL_TARGET',
the following targets will be defined for the whole build system:
 - Copy<GLOBAL_TARGET_NAME>PythonResourceFiles
 - Copy<GLOBAL_TARGET_NAME>PythonScriptFiles
 - Compile<GLOBAL_TARGET_NAME>PythonFiles

For complex projects, this can help reducing the number of targets and
simplify the manual rebuild of copy and compile targets.