Alert user of a pending update when “Check for updates” is pressed

[de0u]

If an update has been downloaded and staged, but the user has disabled notifications with sufficient vigor, the update process will be stuck. If the user discovers via external means that the device is behind, and presses the Check for updates button, nothing happens. This is a recurring source of confusion for users (example1 example2 example3 example4).

Multiple approaches are possible, e.g., Check for updates could reenable notifications, or coach the user to do so. But the simplest thing would be to pop up an alert telling the user that rebooting is necessary to install a new release.

[de0u]

Since #107 was reverted and users are still running into this issue (forum thread) it might make sense to re-open this issue.

Also I wonder if #102 might be useful as an interim measure.

[de0u]

Just to update, there is another user report that the "Check for updates" button "appeared broken" because tapping it had no result. I believe @haok1402 has finished rebasing #102.