First step toward unicode support

In file handling (upload file, file response, ... session, cms, configuration)
Using new classes UTF_CONVERTER, STRING_TABLE and PATH
Using SHARED_EXECUTION_ENVIRONMENT to avoid creating too many instance of EXECUTION_ENVIRONMENT.
Added WSF_PERCENT_ENCODER in wsf/implementation to have an optimized utf8+url encoding.

As decided, the minimum requirement about the compiler is now EiffelStudio 7.2
 (which adds unicode support via STRING_32)

Note there are still some work to do on the meta_variable / environment variable to achieve for unicode support.