Move the line that adds the external page definions into its own
separate include file.
This prevents the files from being included in every Qt
documentation module by default, needlessly duplicating the
information. Currently only the QtDoc module (and modules that
depend on QtDoc) need it.
Task-number: QTBUG-41003
Change-Id: Ie2ddd3a645ea731787daacfffb3068cb36c79c9a
Reviewed-by: Martin Smith <martin.smith@digia.com>
Reviewed-by: Venugopal Shivashankar <venugopal.shivashankar@digia.com>
