Bug: Keyboard control behavior of the StylableDateTimePicker control

[Nockiro]

It is currently not possible to change the date of a StylableDateTimePicker reliably via keyboard commands as well as via mouse (for the date area you want to change).
Most of the time, the only control you have is over the current day part and even this seems to not always be the case if you play long enough with the control via keyboard.

We should try to fix this behavior/adjust it to the standard controls to avoid confusion and accessibility issues.