[nemo-qml-plugin-calendar] Add timezone bindings for events. Contributes to TJC#81380
The time zone of the event is binded to the system time zone. Which means that when travelling abroad, creating an event for the home zone is not possible and it requires adjustments and calculation to setup the time correctly.
This MR is adding binding support to expose the TimeZone time spec to QML and the name of the zone, if any. It also adds a support to save a different zone than the system one.
This is a WIP, I would like to add tests for it and continue further testing on device. @chriadam, @pvuorela or @jpetrell you may still have a look when you have time if you're interested. It's more or less functional.