Export C++ symbols part 2

[darbyjohnston]

Fixes #1919

This is a replacement for PR #1920

These changes are based on the recommendations from @meshula for exporting symbols without using the CMake GenerateExportHeader functionality.

This PR is a WIP, I've just done the minimum amount of exporting to try out the process. With these changes my test app works OK, without them I get this exception as mentioned in the issue:

libc++abi: terminating due to uncaught exception of type std::bad_any_cast: bad any cast

To do / questions:

• Should the export.h file be shared by the opentime and opentimelineio libraries, or should each library get it's own export.h file? (EDIT: I added a separate export.h file for opentime and opetimelineio, with some shared functionality in the opentime export.h)
• The AnyDictionary class exposes some symbols with the using keyword; can these be exported or do they need to be replaced with function wrappers? (EDIT: It looks like this is not necessary since everything is inline)
• For the opentimelineio defines I used OTIO_API for brevity (instead of OPENTIMELINEIO_API)

My test app for reference:

#include <opentimelineio/marker.h>
#include <opentimelineio/timeline.h>

int main(int argc, char** argv)
{
    OTIO_NS::SerializableObject::Retainer<OTIO_NS::Timeline> timeline(new OTIO_NS::Timeline);
    OTIO_NS::SerializableObject::Retainer<OTIO_NS::Track> track(new OTIO_NS::Track);
    timeline->tracks()->append_child(track);
    OTIO_NS::SerializableObject::Retainer<OTIO_NS::Marker> marker(new OTIO_NS::Marker);
    OTIO_NS::AnyDictionary md;
    md["type"] = static_cast<int64_t>(23);
    marker->metadata()["kdenlive"] = md;
    track->markers().push_back(marker);
    if (!timeline->to_json_file("test.otio"))
    {
        return 1;
    }
    return 0;
}

The CMake file:

cmake_minimum_required(VERSION 3.3)
project(test)

set(CMAKE_CXX_STANDARD 17)

find_package(OpenTimelineIO REQUIRED)

set(CMAKE_CXX_VISIBILITY_PRESET hidden)

add_executable(test test.cpp)
target_link_libraries(test OTIO::opentimelineio)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.00%. Comparing base (c0e97b0) to head (cd7fb4d).
⚠️ Report is 73 commits behind head on main.

❌ Your changes status has failed because you have indirect coverage changes. Learn more about Unexpected Coverage Changes and reasons for indirect coverage changes.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1953      +/-   ##
==========================================
+ Coverage   84.11%   85.00%   +0.88%     
==========================================
  Files         198      181      -17     
  Lines       22241    13074    -9167     
  Branches     4687     1206    -3481     
==========================================
- Hits        18709    11113    -7596     
+ Misses       2610     1778     -832     
+ Partials      922      183     -739     

Flag	Coverage Δ
py-unittests	85.00% <ø> (+0.88%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/opentime/errorStatus.h	100.00% <ø> (ø)
src/opentime/rationalTime.h	95.83% <ø> (+1.15%)	⬆️
src/opentime/timeRange.h	98.36% <ø> (-0.74%)	⬇️
src/opentime/timeTransform.h	100.00% <ø> (+8.33%)	⬆️
src/opentimelineio/anyDictionary.h	96.92% <ø> (+5.39%)	⬆️
src/opentimelineio/anyVector.h	91.42% <ø> (+7.55%)	⬆️
src/opentimelineio/color.h	41.66% <ø> (ø)
src/opentimelineio/composable.h	100.00% <ø> (ø)
src/opentimelineio/composition.h	83.87% <ø> (+18.35%)	⬆️
src/opentimelineio/effect.h	83.33% <ø> (-16.67%)	⬇️
... and 16 more

... and 115 files with indirect coverage changes

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