shell: use the "view" style class for the main viewport

Currently, gnome-themes-standard sets a white background on all
GtkViewport widgets.
We want to stop rendering one by default, so add a style class to the
main viewport that ensures it gets the same background as the icon
views it packs inside.

https://bugzilla.gnome.org/show_bug.cgi?id=689700

shell/shell.ui

@@ -129,6 +129,9 @@
                     <property name="visible">True</property>
                     <property name="resize_mode">queue</property>
                     <property name="shadow_type">none</property>
+                    <style>
+                      <class name="view" />
+                    </style>
                     <child>
                       <object class="GtkVBox" id="main-vbox">
                         <property name="visible">True</property>