Auto-update start/end in a better way

Start and end times auto-updates during event modification can be improved:

• when changing start time, the end must be moved accordingly preserving current duration, for example if we have an event from 12:30 to 13:30, changing the start time to 13:00 must move forward the end time to 14:00; the duration (1 hour) it's preserved. Keep attention to the situation when also the end date should be moved.
• when changing start date, end must be re-calculated keeping duration
• when changing end date, start needs re-calculated keeping duration only if chosen end date is before actual start