kcms/time: Overhaul UI

The old UI was very confusing to use, I changed it so that:
- Dialogs are used for the time and date picker, with user confirmation for changes
- Tumblers are not used for the time picker, use the time picker component from kclock
- Removed seconds selector (since it was always wrong)
- Fixed the time zone selector (it didn't work before)
- Properly support 24 hour time in the time picker

Fixes plasma/plasma-mobile#286