QtHelp: Reject keywords referencing unknown files

When reading a help file, a keyword referencing a file id that
doesn't belong to any of its files was made to reference its first
file. When registering, the file ids of the keywords weren't checked
either, so an out-of-range id made the keyword reference a file of
other, previously registered documentation.

Mark an unknown file id as invalid when reading, and refuse to
register documentation containing keywords with file ids outside of
its own files.

Task-number: QTBUG-150688
Pick-to: 6.12
Change-Id: I217032b3d15b0d57967b430f58481ebe91de5949
Reviewed-by: Kai Köhne <kai.koehne@qt.io>