If the directory output fields were both left empty but the directory search fields were not, we will now use the directory search fields for the directory output fields.
It is no longer possible to give a directory (be it non-existent or not) in the output file field.
Add forgotten change in previous commit and fix grammar typo in one error.
Fix two logic mistakes in my previous commits.
Implement a checkbox to determine if you want to create a new folder in which the new merged_*.* files will be created.