Added acadoCreateFolder function.

... Works on UNIX, should be tested on Windows+MSVC.
Polishing ExportModule UI structure...

- Added field <name>, in order to be able to prefix the exported files.
- Minor cleaning in the mpc_export class.
Added addition operator to the String class
Updated ExportModule class

- removed setCommonHeaderName() function. setName() of the same class
will update the commo header name, if needed. This is on the TODO list.
Added exportFolderName field to the ExportModule class...