Fix access to the MAXPATHLEN macro

Using std::filesystem::is_* function may throw exception [1].
Use the same functions with the std::error_code parameter to fix
the problem. (closes: #21).

https://developercommunity.visualstudio.com/t/stdfilesystemis-directory-fails-with-exception-if/787098

Avoid clear observation vectors (size = 0, capacity = 1024 default)
during clear node editor function to reuse this vector.