Suggestions for enhancing FileOpenItem, FileSaveItem, FileDirectoryItem classes

Issue originally submitted here: https://code.google.com/p/guidata/issues/detail?id=37

Reported by maurobio, Oct 18, 2014
1. Include a "notempty" parameter, allowing to control if the field can be empty or not
2. Include an "icon" parameter, allowing to set an icon different from the defaults provided
3. Modify the "help" parameter, allowing to entirely override the messages presented by default 
