Add PluginInfo.json and refactor project structure

[alxvth]

• Refactor files such that each plugin's source resides with it's own CMakeList.txt, in preparation for...
• Add PluginInfo.json for each target (loader plugin)
• Specify CXX as language in cmake
• Safer cmake checks
• Only set specific global CMake flags when project in top level (CMAKE_CXX_FLAGS_RELEASE_INIT and CMAKE_SHARED_LINKER_FLAGS_INIT)
• Removed unused H5Utils2.* files