#57 external close calendar window don't set dc->calendar_window

[RPaciorek]

When calendar is close from window manager (e.g. Alt+F4) dc->calendar_window is not set to NULL, As a result of this:
1) tooltips don't work
2) to open calendar need two click on clock
This patch add handler for "delete-event" signal where is destroy callendar window and set dc->calendar_window to NULL.

[RPaciorek]