Fetcher timestamp

[stefan-kolb]

Fixes #3074.

[koppor]

+1 for separating a preference class. I am unsure, whether we introduced static getters. Our current documentation in https://github.com/JabRef/jabref/wiki/Code-Howtos#how-to-work-with-preferences is somewhat shortcoming in that regard. I think, LayoutFormatterPreferences are a good example about our new way to handle preferences.

I think, the only change required is to let JabRefPreferences have a method getTimestampPreferences.

[stefan-kolb]

Ok, I tried to create a own preference class. Not sure if I've done that correctly (as intended at the moment). Should I also replace calls to the preferences inside the General tab?