Notify before a scheduled backup, giving the option to cancel and maybe to delay

[t-m-w]

Before performing a scheduled backup, notify that it is about to occur. Offer the ability to cancel that particular scheduled run, and possibly to delay it.

I'm not sure how much time is appropriate for a warning - I could see as low as 30 seconds, as high as 2 minutes, maybe?

Would be helpful if the notification could "count down" but having one at all would be nice.

[grote]

Is this only to help with #778 or are there other use-cases?

[t-m-w]

Mostly #778 but there could be other times that a backup is not appropriate, such as if you're on a particular Wi-Fi network which may not be metered but is far enough from home that you'd rather wait, or if you know you're on flaky Wi-Fi, or if you want to reboot your phone, or you're about to need to go into Airplane Mode, etc.