First time start: Ask for file directory

[koppor]

We aim for configuration defaults which don't require any manual adjustment in 90% of the cases. The only thing which has to be configured is the "Main file directory" at "External programs".

I think, JabRef should ask for that directory at the first start. Possibly just by a dialog:

Your main file directory is not configured yet. 
[configure now] [skip configuraiton]

If "configure now" is pressed, the preference dialog is opened and the category "External programs" is selected.

Maybe the configuration tab should be split into three: "External programs" should be kept as is, "File handling" (instead of "External file links") and "Sending of emails"

[koppor]

Integrated at https://github.com/JabRef/jabref-issue-melting-pot/issues/49