Currently, the welcome tab offers "New library" and "Open existing"

It misses "Open example library"

Task:

1. Move Chocolate.bib to jabgui/src/main/resources Keep Chocolate.bib in jablib/src/main/resources to enable using in JabKit, too.
2. Add button "Open example library" to Welcome Tab
3. On click: contents of "Chocolate.bib" should be opend in a new library. Reason: Chocolate.bib is saved in JabRefs program folder, which is not user-writable. Thus, the people should be guided to save the library in another file - if they want to continue working.