Build header context menu dynamically when requested

Previously, the list view's header context menu was created and filled
at initialization time. However, that would not account for changes
of bibliography (BibTeX <-> BibLaTeX) which bring different columns
and as such different context menu items.

Now, the context menu is built upon request (e.g. right mouse click)
and filled and configured based on current BibTeXFields instance
and list view's column visibility.