doxygen: parse override, final and conditional noexcept keywords.

doxygen: parse override, final and conditional noexcept keywords.

 * override and final is now parsed from function signatures, in the
   theme the [virtual] label gets replaced with either [override] or
   [final], since it doesn't make sense to include both [virtual] and
   [override]/[final] label as the former is implicit for the latter.
 * fixed order of parsing function signature keywords so function
   signatures that are both deleted/defaulted and noexcept are parsed
   correctly instead of noexcept being ignored
 * conditional noexcept is now recognized as well, shown in the theme as
   [noexcept(...)]
 * [final] class specifier is now shown in the class header and also in
   the derived class list
 * [virtual] label is now shown for both virtual base class and
   virtually derived classes to make it symmetric