qdoc: Further improve example file listing

Fix a bug where '\generatelist examplefiles' with no second parameter
produced an empty list, instead of listing all files.

Also, no longer restrict the searching of example image files to the
images/ subdirectory. All images under a project directory are now
listed, except those under doc/images.

Minor fixes to related QDoc documentation.

Change-Id: Id4516ba2feb98466a0e893a85f85118e3d21eae2
Reviewed-by: Martin Smith <martin.smith@qt.io>
Reviewed-by: Venugopal Shivashankar <Venugopal.Shivashankar@qt.io>