Vertically expand the source selector

Resizing the source selector window will also expand
the scrolled window. Without this change the scrolled
window remains the same height no matter how tall the
window is.

glade/node_source.ui

@@ -84,6 +84,7 @@
               <object class="GtkScrolledWindow" id="scrolledwindow1">
                 <property name="visible">True</property>
                 <property name="can_focus">True</property>
+                <property name="vexpand">True</property>
                 <property name="hscrollbar_policy">never</property>
                 <property name="shadow_type">in</property>
                 <child>