DoxygetTargets: switch to CMakeParseArguments macro

This gets rid of the manual argument parsing and uses CMake's own macro
for this task.
DoxygenTargets: add optional argument QUIET
DoxygenTargets: add optional argument WORKING_DIRECTORY

This is the directory in which Doxygen will be executed. If the Doxygen
configuration variable INPUT contains relative paths, WORKING_DIRECTORY
will be used as the base path.
DoxygenTargets: update help
Merge branch 'doxygentargets-update-and-extend'