Improve update check

[stoically]

• Clicking the "check for updates" button in riot web could also call runtime.requestUpdateCheck to check for new Add-on version. Though, it would be preferable if the interval update check in riot web would not call the runtime API.

• If onUpdateAvailable fires and no riot tab is open the Add-on should reload immediately

• Riot tabs should communicate with the background with connect rather than sendMessage, that would avoid having to depend on setTimeout