#52 Confirm with user if output target exists

[Joel Håkansson]

Check if the selected output file exists or folder contains files as soon as the output dialog box is about to be closed (user pressed "Save" (file) or "OK" (folder)).

If file exists or folder isn't empty, inform user that files could be overwritten and confirm that this is OK.

Background:
Currently, each transformer has to perform its own check if files will be overwritten and decide what to do (the only options are to overwrite or fail and report an error as no interaction with the user is possible at this point). Not only does the user have to go through the process of creating a new job with different settings, each script behaves differently in this situation, making it hard for the user to predict behavior.