Toggle completion of occurrences of recurring todos

Currently the 'Toggle To-do Completed' action of the month and agenda views, applied to a recurring todo, marks the next two incomplete occurrences as complete, regardless of which occurrence was selected.

This patch calculates the selected occurrence's recurrence ID, and toggles that occurrence. If it was incomplete, it and all previous occurrences are complete; if it was complete, it and all following occurrences are incomplete.