Remove unsafe interface requirement for into ptr conversion

[BiagioFesta]

Fixes: #1663

For reviewers to focus on

Beside "sed changes", manual changes also include:

• impl IntoGlibPtr<*mut c_char> for GString requires inner unsafe block as calling extern "C" functions
• pub unsafe trait RefCounted::into_raw also changed removing unsafe requirement.
• This is a API (breaking/major) change

[sdroege]

Thanks!