UTF-8 not preserved in some API calls on Linux and Windows

[limbonaut]

In some places such as set_tag() and get_tag(), get_tag() would not preserve UTF-8 encoding, returning a different string. This does affect mostly getters, not setters.

When we take C strings from native SDK and convert to Godot strings, we need to use String::utf8 where those C strings realistically can contain UTF-8 encoded data.

[linear]

GODOT-101 Issues with UTF-8 not preserved in some APIs on Linux and Windows