when adding a caledar event pre-select current time

[kibibytium]

Currently when adding a calendar event by clicking on the plus button, the time is set to from 00:00 to 00:30 on the current day.

It would improve usability to have the current time of day instead.

Test notes

• Creating an event from the New Event button should open the event editor with the time set to the next half hour
• Creating an event from the Month view by clicking on a day should open the event editor with the selected day, and the time set to the next half hour

Next half hour rule:

• If it's 14:12, next half hour will be 14:30 ~ 15:00
• If it's 14:35, next half hour will be 15:00 ~ 15:30