Add a duration picker dialog

[Namnodorel]

It would be nice to have a duration dialog that allows picking arbitrary durations (like 4 hours, 15 minutes, etc.).

I think a lot of the time-picker UI could be reused for this. There would be no need for AM/PM or differentiating between 12/24-hour formats, but having two text fields for hours and minutes (maybe optionally one for days as well) with the same design would work well.