apps-menu: Make it a bit narrower

General rule: if it can fit "GNU Image Manipulation Program", it is
wide enough ...

https://bugzilla.gnome.org/show_bug.cgi?id=745670

extensions/apps-menu/extension.js

@@ -476,7 +476,7 @@ const ApplicationsButton = new Lang.Class({
 
     _display: function() {
         this._applicationsButtons = new Array();
-        this.mainBox.style=('width: 640px;');
+        this.mainBox.style=('width: 35em;');
         this.mainBox.hide();
 
         //Load categories