UnderlyingValue deprecated warnings in release 1.1.0 Testbed build

[louis-langholtz]

Expected/Desired Behavior or Experience:

Testbed builds without warnings about UnderlyingValue being deprecated in release 1.1.*.

Actual Behavior:

Seeing warnings like:

[ 91%] Building CXX object Testbed/CMakeFiles/Testbed.dir/Framework/Main.cpp.o
PlayRho/Testbed/Framework/Main.cpp:1182:28: warning: 'UnderlyingValue<unsigned short, playrho::BodyIdentifier>' is deprecated: Use
      to_underlying instead [-Wdeprecated-declarations]
    ImGui::IdContext idCtx(UnderlyingValue(b));
                           ^
PlayRho/PlayRho/Common/IndexingNamedType.hpp:142:3: note: 'UnderlyingValue<unsigned short, playrho::BodyIdentifier>' has been explicitly
      marked deprecated here
[[deprecated("Use to_underlying instead")]]
  ^
PlayRho/Testbed/Framework/Main.cpp:1353:28: warning: 'UnderlyingValue<unsigned short, playrho::FixtureIdentifier>' is deprecated: Use
      to_underlying instead [-Wdeprecated-declarations]
    ImGui::IdContext idCtx(UnderlyingValue(fixture));
                           ^
PlayRho/PlayRho/Common/IndexingNamedType.hpp:142:3: note: 'UnderlyingValue<unsigned short, playrho::FixtureIdentifier>' has been
      explicitly marked deprecated here
[[deprecated("Use to_underlying instead")]]
  ^

Steps to Reproduce the Actual Behavior:

git clone --recurse-submodules --branch release-1.1.0 https://github.com/louis-langholtz/PlayRho.git
mkdir PlayRho-build
cd PlayRho-build
cmake -DPLAYRHO_BUILD_TESTBED=ON ../PlayRho
make