Fixed DateTimeElement showing a non centered UIDatePicker if device in landscape

When a DateTimeElement is selected for edition and the device is in landscape mode, the new DialogViewControl displayed for edition shows a UIDatePicker not centered in the middle of the screen.
Auto rotation while in edition mode is still functional.

PS: first time with github so please forgive me for the pull request including my merge, I was not able to create a pull request with omitting it...