imgui-sys: Low level bindings

This crate contains the raw FFI bindings to the Dear ImGui C++ library, by using the cimgui (a C API wrapper project for Dear ImGui), then creating Rust bindings using bindgen.

These low level, mostly unsafe bindings are then used by imgui-rs which wraps them in a nice to use, mostly safe API. Therefore most users should not need to interact with this crate directly.