Handle failed attempts to split a side window (Bug#18304).
* window.el (window--side-window-p): New function. (split-window, window-splittable-p): Use window--side-window-p to determine whether WINDOW can be split (Bug#18304). * calendar/calendar.el (calendar-basic-setup): Fix one call of `window-splittable-p' and add another (Bug#18304).
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