Revamp update checking / applying / downloads

[fasterthanlime]

Checking for an update (or downloading one) shouldn't block playing a game. Applying the update is the only part that should block launching the game.

Potential behavior when clicking play:

• If it currently has an update check queued (e.g. on app start), it'll bump the priority of the update check to the top of the list, while allowing you to 'ignore it'.
• If an update is downloading (or pending download, depending on your settings), it should ask you if you want to wait for the latest version, then launch, or if you just want to launch now.

Downloads should set progress globally so it appears in OSX dock / Windows progress bar / Unity dock, one should be able to re-order downloads by priority easily (by drag & dropping, or buttons)

[ghost]

Yes that should be done (:

[fasterthanlime]

Checking for an update (or downloading one) shouldn't block playing a game.

That part is done! cf. other issues like #158 #609

[fasterthanlime]

Downloads have been vastly improved in v18 (pause/resume, prioritizing)