Use slider for nlocktime block selection

[greenaddress]

Use discrete values (10 days, 30, 60, 90, 180, 250, 300, 350, 400, 500) in the slider and set to block selection.

UI only change, API still allows all kinds of settings.

If value is not withing range (already set to say 200 blocks manually by user) then just show that and hold the slider on that position, don't break the UI!