Update version to 3.0.0

• Add
  External linkage support for c++ projects
  Replace list parse engine with map based one
  Roxml_commit_file(), roxml_commit_fd() and roxml_commit_buffer() helpers
  Subtree display too roxml tool
  Rocat tool
  Optional disabling support of float XML values
  Basic XML validation during parsing
  Header for all the source files updated to SPDX
  Parser function locality
  Global code refactor
  Add support for XPath function local-name()
  Update documentation
  .
• Bugfixes
  Fix tests
  Fix various compilation warnings
  Fix potential overflow and memory-leak related to namespace parsing
  Fix CMake builds
  Fix recursive delete of malformed tree
  Fix wrong calculation for buffer realloc during commit
  Fix spurious colon in default namespace