refactor(common): de-Qt oakcommon and wrap it in a pure C ABI

- 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

src/common/tests/CMakeLists.txt

@@ -0,0 +1,44 @@
+# Olive - Non-Linear Video Editor
+# Copyright (C) 2022 Olive Team
+# Modifications Copyright (C) 2025 mikesolar
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+find_package(GTest REQUIRED)
+include(GoogleTest)
+
+add_executable(oakcommon-gtest
+        commandlineparser_test.cpp
+        current_test.cpp
+        debug_test.cpp
+        dropworkflowbehavior_test.cpp
+        ffmpegutils_test.cpp
+        filefunctions_test.cpp
+        memorypool_test.cpp
+        miscutils_test.cpp
+        ocioutils_test.cpp
+        oiioutils_test.cpp
+        power_test.cpp
+        qtutils_test.cpp
+        threadsafemap_test.cpp
+        xmlutils_test.cpp
+)
+
+target_link_libraries(oakcommon-gtest PRIVATE
+        oakcommon
+        GTest::gtest
+        GTest::gtest_main
+)
+
+gtest_discover_tests(oakcommon-gtest)