message(): Add support for log levels

Auxiliary/bash-completion/cmake

Help/command/message.rst

Help/manual/cmake.1.rst

Help/release/dev/new-message-types-and-logging.rst

Source/cmMessageCommand.cxx

Source/cmake.cxx

Source/cmake.h

Source/cmakemain.cxx

Tests/RunCMake/message/RunCMakeTest.cmake

Tests/RunCMake/message/message-all-loglevels.cmake

Tests/RunCMake/message/message-loglevel-debug-stderr.txt

Tests/RunCMake/message/message-loglevel-debug-stdout.txt

Tests/RunCMake/message/message-loglevel-default-stderr.txt

Tests/RunCMake/message/message-loglevel-default-stdout.txt

Tests/RunCMake/message/message-loglevel-invalid-result.txt

Tests/RunCMake/message/message-loglevel-invalid-stderr.txt

Tests/RunCMake/message/message-loglevel-notice-stderr.txt

Tests/RunCMake/message/message-loglevel-status-stderr.txt

Tests/RunCMake/message/message-loglevel-status-stdout.txt

Tests/RunCMake/message/message-loglevel-trace-stderr.txt