Adding multiple folders to be searched for duplicates

[71walceli]

Enable to add multiple folders to search and to exclude

• Edited fslint-gui.on_addOkDir_clicked()
• Edited fslint-gui.on_addBadDir_clicked()
• Edited fslint.gradeline 3922: set select_multiple to true
• Extracted a constant for bad dirs

I hope that you find these improvements useful.