visualizer-selection: initial visualizer selection support

This gets started on allowing the user to select a region.
Nothing is yet responding to the changes in selection, but
that can come later.

Ideally, we will truncate (or mark rows invisible/insensitive)
if they do not fall within the selected time region.

lib/resources/ui/sp-visualizer-view.ui

@@ -10,6 +10,9 @@
             <property name="hexpand">true</property>
             <property name="visible">true</property>
           </object>
+          <packing>
+            <property name="pass-through">true</property>
+          </packing>
         </child>
         <child>
           <object class="GtkScrolledWindow" id="scroller">