documentation/doxygen: add support for class-specific location.

documentation/doxygen: add support for class-specific location.

When documenting a class, a struct or an union, Doxygen allows the user
to specify a custom header.

Doxygen doc:
  \class <name> [<header-file>] [<header-name>]
  \struct <name> [<header-file>] [<header-name>]
  \union <name> [<header-file>] [<header-name>]

The specified <header-name> can be retrieved between the
<includes></includes> tag of the generated xml for the
classes/structs/unions constructs.

This patch uses <header-name> instead of the default path extracted from
the <location file="xx"/> without modifying the location.

This also fixes #137 for these three constructs when the user does not
define the <header-file>/<header-name>, because the value present inside
the <includes> tag is stripped using the Doxygen STRIP_FROM_INC_PATH
option, whereas <location file="xx"/> is not. Hence, there is still an
issue for namespaces, free functions, enums, typedefs, variables and
defines which are not part of a class/struct or union.

Co-authored-by: Vladimír Vondruš <mosra@centrum.cz>