Problem: Doxygen runs with QUIET=NO and
WARN_LOGFILE=@CMAKE_CURRENT_BINARY_DIR@/doxyerror.log. This writes all
Doxygen errors and warnings to doxyerror.log instead of to
stderr. However, errors from external tools called by Doxygen, such as
PlantUML and dia, are not included. These are only writen to stderr, and
lost in a long list of Doxygen stdout messages.

Solution: Use the output to stderr instead of WARN_LOGFILE in order to
pick up errors from third party tools. Build all error filtering that
used to be done with sed and grep into the CMake script, so that
everything is done by one call to make doxygen.

The result is the following files:

doxyoutput.log:
  stdout from Doxygen.

doxyerror.log:
  stderr from Doxygen.

tofix-all.log:
  Same as doxyerror.log, but without dia status messages, and with paths
  relative to the source directory.

tofix-regressions.log:
  Same as tofix-all.log, but without known warnings and errors (filters
  defined in Doxyfile-ignored).

The tofix-regressions.log messages are also printed to stdout during
make doxygen, and the target will finish by reporting if any regressions
were found.

Change-Id: I0e45745976bedbcb83f8ed952e52f927de99eb21