add a checkbox to start the game right after being downloaded

[illwieckz]

Right under the download bar in main ui we may add a checkbox to tell the updater to automatically run the game once download finishes.

In options, we may put another checkbox to automatically download updates once available. With the updater displaying the main ui with the news excerpt during max(download time, arbitrary time) when both autodownload and autostart is activated.

[slipher]

In my opinion, it would be too disruptive to suddenly start an (often) full-screen application like Unvanquished.

To notify users that the download is finished, how about QApplication::alert?

[illwieckz]

To notify users that the download is finished, how about QApplication::alert?

Good idea if this implementation is well integrated.