Add initial recurrence support implements, #462 (#814)

This commit adds initial recurrence support in that recurring instances are handled correctly when instances are edited or deleted.
In order to support this, the UI now operates on the Instances table rather then the tasks table. The instances table now supports deletes, and updates like the tasks table with the difference that all these operations only affect the specific instance that's addressed. All the operations on the instances table are converted into operations on a task by creating exception instances and RDATEs or EXDATEs.

Inserting instances is currently not possible because only a fixed number of instances are expanded. This means an instance inserted beyond the expansion window would still not show up in the instances table.
See https://github.com/dmfs/opentasks/issues/741

There is still room for improvement in that we could split of completed tasks and modify recurrence rules when deleting instances from the start or end of the series. That's left to future updates.
Also there is no UI yet to create or edit recurrence properties itself.

At present there is only one upcoming instance expanded, so you can only see the "current" instance (and all completed ones). This may/will change in future but may require some UI changes as well.

In order to support this commit a number of tests have been implemented which test creating and altering recurring instances.

Currently an exception is created when a recurring instance is modified which wasn't an exception before. This approach is RFC 5545 compliant but not supported by Apple clients. This will be fixed in future updates.