Classification fails without clear feedback if output is not a file

[kamilzyla]

Problem

There are two issues related to "Select output directory" field on the classification page:

1. This line can fail to get user's Documents directory, in which case no "Save as" dialog is shown and no error is displayed.
2. The hint text says "Select output directory", but it actually needs to be a file - otherwise Mbaza will fail to save results without clear indication what happened ("Classification failed").

Workaround

It is possible to work around this problem by manually pasting path into the output field. If the file doesn't exist, it will be created. If it does, it needs to be a regular file (not a directory), in which case it will be overwritten.