Improving starting app handler

[jsilvestre]

As a user, when I browse a stopped application,
I want to see a spinner with a message during the application start
And be redirected to the application once it's ready

That could be achieved that way:

• when the app_manager receives a startApp request, it doesn't wait for the response and instead sends a "please wait" template
• when the app is actually started, it sends a realtime event to the "please wait" template that triggers a refresh

I'm not sure this is an actual improvements in every case (i.e. not for slow connection) so I'd suggest we only do it for desktop devices.