[nemo-qml-plugin-calendar] Avoid segfault on shutting down application

CalendarDb has static instances of mkcal calendar&storage. Those have
then static instances of KTimeZone etc. Need to explicitly clear references
so deletion order is deterministic.
Merge pull request #19 from pvuorela/master

[nemo-qml-plugin-calendar] Avoid segfault on shutting down application