baseIcon: Stop using Shell.GenericContainer

Pretty much like the previous patches, this extends St.Bin. The
most interesting aspect of this patch is that most of the sizing
routines of the icons is now delegated to the actors and layout
managers, removing quite a bunch of code.

The 'spacing' theme property is now redirected to StBoxLayout's
spacing property. Also adjust the Dash code to stop forcing a
potentially invalid width in the first icon too.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/153

js/ui/search.js

@@ -147,7 +147,7 @@ var GridSearchResult = new Lang.Class({
 
         this.icon = new IconGrid.BaseIcon(this.metaInfo['name'],
                                           { createIcon: this.metaInfo['createIcon'] });
-        let content = new St.Bin({ child: this.icon.actor });
+        let content = new St.Bin({ child: this.icon });
         this.actor.set_child(content);
         this.actor.label_actor = this.icon.label;
     }