Theme: add helper class names for Adwaita

- helps legibility of app icons

Addresses https://gitlab.gnome.org/GNOME/gtk/issues/1434

gtk/ui/gtkaboutdialog.ui

@@ -33,6 +33,9 @@
             <property name="spacing">12</property>
             <child>
               <object class="GtkImage" id="logo_image">
+                <style>
+                  <class name="icon-dropshadow"/>
+                </style>
                 <property name="visible">1</property>
                 <property name="icon-name">image-missing</property>
               </object>

gtk/ui/gtkappchooserwidget.ui

@@ -75,7 +75,12 @@
                       <object class="GtkCellRendererText" id="secondary_padding"/>
                     </child>
                     <child>
-                      <object class="GtkCellRendererPixbuf" id="app_icon"/>
+                      <object class="GtkCellRendererPixbuf" id="app_icon">
+                        <style>
+                          <class name="lowres-icon"/>
+                        </style>
+                        <property name="stock-size">5</property>
+                      </object>
                       <attributes>
                         <attribute name="gicon">1</attribute>
                       </attributes>