util: add Util.spawn and friends

Add Util.spawn, Util.spawnCommandLine, and Util.spawnDesktop for
spawning a command/argv/.desktop file in the background, automatically
handling errors via MessageTray.SystemNotificationSource(), and
Util.trySpawn, Util.trySpawnCommandLine, and Utils.trySpawnDesktop
that don't do automatic error handling (but do at least clean up the
error message in the exception a bit).

Update various other bits of code around the shell to use the new
methods.

https://bugzilla.gnome.org/show_bug.cgi?id=635089

js/ui/messageTray.js

@@ -106,8 +106,7 @@ URLHighlighter.prototype = {
                     return true;
                 } catch (e) {
                     // TODO: remove this after gnome 3 release
-                    let p = new Shell.Process({ 'args' : ['gvfs-open', url] });
-                    p.run();
+                    Util.spawn(['gvfs-open', url]);
                     return true;
                 }
             }