Save active view on the todo panel

Add a configuration option appearance.todoview and use it to
automatically save the currently active todo panel view and restore it
when restarting calcurse.

Signed-off-by: Lukas Fleischer <lfleischer@calcurse.org>

src/calcurse.h

@@ -1072,6 +1072,8 @@ void ui_todo_view_note(void);
 void ui_todo_edit_note(void);
 void ui_todo_view_prev(void);
 void ui_todo_view_next(void);
+int ui_todo_get_view(void);
+void ui_todo_set_view(int);
 
 /* utf8.c */
 int utf8_width(char *);