pathbar: do not unconditionally bubble up button-press-event

https://bugzilla.gnome.org/show_bug.cgi?id=706605
pathbar: add a return value from path-event signal

And make it so the return value is actually given in the callback,
instead of depending on the window and the pathbar agree on what
happened.

https://bugzilla.gnome.org/show_bug.cgi?id=706605