Research how should we use get_preferred_size

[manuq]

​http://developer.gnome.org/gtk3/stable/GtkWidget.html#gtk-widget-get-preferred-size

It returns two requisitions, minimun size and natural size. Which should we use? Are we doing ok in ​here, using the width of one and the height of the other?:

Also, move pending do_size_request to do_get_preferred*. These are:

./src/jarabe/journal/listview.py:53:    def do_size_request(self, requisition):
./src/jarabe/journal/listview.py:60:            Gtk.TreeView.do_size_request(sel
./src/jarabe/journal/listview.py:233:    def do_size_request(self, requisition):
./src/jarabe/frame/framewindow.py:68:    def do_size_request(self, req):