dropdown: Style fixups

Make the dropdown popover have a .menu style class,
and add padding to the rows in the popover. Also,
make the popover left-aligned with the button.

gtk/theme/Adwaita/_common.scss

@@ -1173,6 +1173,10 @@ combobox {
   }
 }
 
+dropdown > popover listview > row {
+  padding: 6px;
+}
+
 // the combo is a composite widget so the way we do button linking doesn't
 // work, special case needed. See
 // https://bugzilla.gnome.org/show_bug.cgi?id=733979

gtk/ui/gtkdropdown.ui

@@ -45,8 +45,12 @@
     <child>
   <object class="GtkPopover" id="popup">
     <signal name="closed" handler="popover_closed"/>
+    <property name="halign">start</property>
     <property name="has-arrow">0</property>
     <property name="autohide">1</property>
+    <style>
+     <class name="menu"/>
+    </style>
     <child>
       <object class="GtkBox">
         <property name="orientation">vertical</property>