Add time-of-day scheduling option to the UI

Simple feature request here. The JSON file allows scheduling backups at a certain time of day; I would appreciate having that ability in the UI. Thanks for the great tool!

Beware that if you’re configuring time of day backups and your client misses it - e.g. a notebook, which has been sleeping over it, you will miss the snapshot. Contrary to the time-based ones, which will run as soon as they’re due and afterwards when the client becomes active.