qdoc: Improve example file listing

At the end of \example documentation page, qdoc lists all the files
that are part of the example project. This commit does the
following improvements:

  - Sort the example files based on path name
  - Introduce '\generatelist examplefiles [regexp]', which
    lists the files (optionally, only those that match regexp)
    at the specified location.
  - Enable \noautolist for example pages, to skip generating the
    the file list at the end.

Change-Id: Ic9ae70f9f7de166d314ae489a7bab9a935949fc3
Reviewed-by: Topi Reiniö <topi.reinio@qt.io>