Draft: edit-dialog: Flexible reminders time delays

Currently there is no way to specify custom time delays when creating reminders.

To add more options, add a new button in alarms popover that launches another popover, in which one can specify value and time unit for the new reminder.

Suggestion: Enter the unit with a (underlined) shortcut, too. In example when I need to set 2 days, I press "2d", or "48h" without need to use the mouse for selecting the unit, and the popover close automatically

Closes #251