Clean-up styling improvements for Calendar

This is a fix-up for 4569ed127c290bea8d165e15dc2eca525825c627 :
"verticalAlignment: Text.AlignVCenter" not needed for alread-centered text;
make previousMonth button center-in parent, similarly to nextMonth button;
drop dead code from HoverButton.

Change-Id: Iceb096944d084ecded70611e3da754150a0e098e
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>