Bug 651850 - @defgroup generates manpage warning

In case the \cite command is used but no entry for it is given in a bib file no warning is given as the CiteInfo entry is present with the right label, but the text item is empty and this is not tested.

(also corrected an error message on opening a file)

Merge branch 'feature/bug_700510' of https://github.com/albert-github/doxygen into albert-github-feature/bug_700510

Changed the previous fix to allow references to sections defined in
a mainpage that is imported via a tag file.

Correction of typing error

Corrected documenation -> documentation

Support multiple extra HTML stylesheets.

In regular use it happens that it is more convenient to have multiple extra HTML stylesheets, e.g. in case of different levels of requirements: company, department, project
With this patch it is possible to specify multiple HTML stylesheets.
Note: for compatibility the configuration parameter HTML_EXTRA_STYLESHEET has not been renamed to HTM_EXTRA_STYLESHEETS.

Bug 733722 - XML Output invalid: XML_PROGRAMLISTING=YES copies Unicode form feed character (U+000C) to XML files

Bug 631675 - Java inheritance of instantiated generics not recognized.

Merge branch 'feature-groupbreadcrumbs' of https://github.com/NimbusKit/doxygen into NimbusKit-feature-groupbreadcrumbs