Exposing bare pointers to immutable variables & members as properties

[dcnieho]

This is about the discussion at #41: Right now we have a working solution for wrapping ImGuiIO.IniFilename, with is a bare const char*. Wrapping that with a simple interface closest to C++ imgui (users can simply get and set through imgui.get_io().ini_filename) would require some work. Either, if thats legal, we'd have to generate:

.def_property("ini_filename", &ImGuiIO::IniFilename, &ImGuiIO::SetIniFilename, "doc string")

or, else

.def_property("ini_filename", [](const ImGuiIO &in) {return in.IniFilename;}, &ImGuiIO::SetIniFilename, "doc string")

This would be a nice to have.