WIP: restructure button handling

When creating a new event, there is currently no "ok" action when setting a recurrence. Confirmation happens with the back action that is placed at the left side of the header.

The new events cancel action is placed at the left side of the header too.

Problem is: When accidentally pressing back twice, the new event gets cancelled, because you press the cancel action of the new event. I did that a few times. :-(

Also it is not common to confirm a selection with the back action. Therefore I propose to add a "ok" button for confirmation and use back button for cancelling.

In addition this MR will make use of a proper header component instead of the old head component. And a comments typo/wording is improved.