Enable 32 bit ImWchar

Recently ImGui introduced an option to enable full unicode support by using 32 bit ImWchar. This requires adding IMGUI_USE_WCHAR32 define in imconfig.h. It would be nice if ImGui.NET switched to use this. This would enable full unicode support, as well as allow using fonts like Material Design Icons that uses U+F0000..U+FFFFD private user plane. That would be really useful.

I suspect that this would be very hard to actually support in C#, because strings are unconditionally UTF16, and thus the interop between regular code and ImGui.NET would become very tricky. How are you envisioning that would work?

This answer and this answer in stack overflow might be of help, but I haven't looked thoroughly, I'm just interested in seeing this becoming possible

@mellinoe do you need help with that? Really annoying and makes it impossible to use some fonts from https://github.com/juliettef/IconFontCppHeaders

@seclerp sure, feel free to create a PR. For @mellinoe reply, please read the readme.md file.