dash: Adjust placeholder size to icon size

The placeholder looks odd near small icons and causes the dash to get wider
when visible and narrower when hidden.

https://bugzilla.gnome.org/show_bug.cgi?id=659210

data/theme/gnome-shell.css

@@ -511,8 +511,6 @@ StTooltip StLabel {
 
 .dash-placeholder {
     background-image: url("dash-placeholder.svg");
-    height: 27px;
-    width: 48px;
 }
 
 #viewSelector {

js/ui/dash.js

@@ -684,6 +684,8 @@ Dash.prototype = {
             }
 
             this._dragPlaceholder = new DragPlaceholderItem();
+            this._dragPlaceholder.child.set_width (this.iconSize);
+            this._dragPlaceholder.child.set_height (this.iconSize / 2);
             this._box.insert_actor(this._dragPlaceholder.actor,
                                    this._dragPlaceholderPos);
             if (fadeIn)