Move the pathbar creation to its own function

Move the pathbar creation to its own function

We will centralize the place where all the pathbar-related widgets are created:
the location button, the pathbar itself, the Create Folder button, and in
subsequent commits, the info bar as well.  We will deal with the pathbar/infobar
as a unit, instead of swapping them in and out in an ad-hoc fashion.

Signed-off-by: Federico Mena Quintero <federico@gnome.org>