iconGrid: Fix icon alignment

We don't want the icon to fill extra space, so set the alignment
accordingly. Otherwise we get an unexpected result when adding
a background just to the icon part (as far as I can tell: just
system-action-icon).

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/909

js/ui/iconGrid.js

@@ -51,7 +51,7 @@ class BaseIcon extends St.Bin {
         this.set_child(this._box);
 
         this.iconSize = ICON_SIZE;
-        this._iconBin = new St.Bin();
+        this._iconBin = new St.Bin({ x_align: Clutter.ActorAlign.CENTER });
 
         this._box.add_actor(this._iconBin);