Add app update button and update status next to app version info

[KrzysztofPrzygoda]

This is small enhancement UX idea.

Checking current app version could provide direct check for update button and info that there is one - same info as check for update dialog provides.

Now I need to go from About slate to Application > Update > Automatically check for updates section to find out if current version is up to date.

[waydabber]

I wanted to change the update mechanism to a nicer inline solution for a long time: #1459

The current intrusive method with annoying popups is not elegant at all, it's just the baseline mechanism provided by Sparkle for lazy app developers. :)