year-view: implement the ::drag-drop operation

This magical commit finally implements the Drag and Drop
on Year view, enabling users to drop events from the sidebar
to the year navigator.

data/ui/year-view.ui

@@ -25,6 +25,7 @@
                 <signal name="button-release-event" handler="navigator_button_release_cb" object="GcalYearView" swapped="yes"/>
                 <signal name="motion-notify-event" handler="navigator_motion_notify_cb" object="GcalYearView" swapped="yes"/>
                 <signal name="drag-motion" handler="navigator_drag_motion_cb" object="GcalYearView" swapped="yes"/>
+                <signal name="drag-drop" handler="navigator_drag_drop_cb" object="GcalYearView" swapped="yes"/>
                 <style>
                   <class name="year-navigator"/>
                 </style>