views: use ACTION_MOVE to display correct cursor

When dragging events around, the cursors show a + as opposed to a
an arrow indicating that the event is being moved. This can confuse
users.

The fix is to tell GTK that the drag being performed is a move.

https://bugzilla.gnome.org/show_bug.cgi?id=774922

src/views/gcal-week-header.c

@@ -1383,7 +1383,7 @@ gcal_week_header_drag_motion (GtkWidget      *widget,
    * aborts the drag with FALSE.
    */
   gdk_drag_status (context,
-                   self->dnd_cell == -1 ? 0 : GDK_ACTION_COPY,
+                   self->dnd_cell == -1 ? 0 : GDK_ACTION_MOVE,
                    time);
 
   gtk_widget_queue_draw (widget);