- de-Qt all classes under src/common (std::string/vector/mutex,
std::filesystem, expat-based XmlStreamReader/Writer)
- add pure C ABI in include/common + src/common/c_api: opaque handles,
init returns NULL on failure, free(NULL) is a no-op, out-params with
negative OAKCOMMON_E_* error codes (include/common/error.h)
- remove single-consumer classes from oakcommon (html, jobtime,
otioutils, playbackaudioclock, tohex, util, avframeptr,
crashpadinterface/crashpadutils, autoscroll, digit, range); their
destinations are recorded in docs/zh/plans/riir/notes.md
- add gtest suites under src/common/tests (127 cases), standalone
build driver in src/common/standalone
