Fixed: custom CPWindow subclasses breaking flatten.

If a cib file had a custom CPWindow subclass, that cib file would not be properly processed by flatten as it'd fail to find the relevant class (the fact that the class can't be found is an issue in itself). This would cause an exception.

This fix makes CPCibWindowTemplate act similar to CPCibCustomView when the referenced class can't be found: it instantiates a regular CPWindow. This is sufficient for flatten's purposes.