Formatting
Fixed: removed deprecated check for no resource path.

Long ago it was required to use the -R option with nib2cib if custom images were used. nib2cib now infers the resource path, so there will always be at least a default resource path. Thus the check for no resource path was unnecessary.

This commit removes that check.
Fixed: shared singleton Converter was being overwritten by incompletely constructed instance.

Previously, NSNib would create a Converter instance for converting table cell views. Unfortunately Converter did not expect to be instantiated more than once, and the singleton SharedConverter was unconditionally being set in the initializer. This might not have caused problems, except that NSNib did not copy all of the shared Converter's state to the copy, especially the resource path, which caused custom images in table cell views to fail.

This commit correctly ensures that the shared Converter instance is set only once, and also copies all of the shared Converter state to the instance that is used for converting the table cell view.