Draft: Show occurrence details in Event Viewer

If the user selects an occurrence of a recurring item in the Agenda or Month views and the Item Viewer is visible in the sidebar, it shows the start date (for events) or start and due dates (for to-dos) of that occurrence. The Event Viewer created from the right-click menu does something less useful: it displays the start date of the first occurrence and the due date of the first incomplete undissociated occurrence, no matter which occurrence is clicked.

This patch passes the occurrence's date down to the Event Viewer to specify which occurrence to display.