calendar.c: Rename to "ui-calendar.c"

This unit belongs to the presentation layer -- rename the file
accordingly.

Also, rename calendar_*() to ui_calendar_*().

Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>

src/utils.c

@@ -80,7 +80,7 @@ void exit_calcurse(int status)
   } else
     was_interactive = 0;
 
-  calendar_stop_date_thread();
+  ui_calendar_stop_date_thread();
   io_stop_psave_thread();
   free_user_data();
   keys_free();