fix(calendar): Trigger onMonthChanged only when month really changed #41

* Clicking another day in the calendar calls _launchDateSelectionCallback
* This function checks if the parameters onDateSelected and onMonthChanged are not null
* If the parameters are not null, they get called
* This means, that onMonthChanged also gets called, when the month did not change

CHANGE

* This is now not possible anymore, because the check now includes a test, if month or year were changed.
  Only then onMonthChanged gets executed.

Fixes #41

#41