unicode inputChar support for imguiBeginFrame

[cloudwu]

I'm writing a bgfx imgui lua bindings, and I found imguiBeginFrame only support ascii char now.

This PR add a unicode inputChar support by io.AddInputCharactersUTF8.

btw, I include <dear-imgui/imgui_internal.h> because I use the internal function ImTextStrToUtf8. I'm not sure is there any equivalent function in bx. (Converting a unicode code point to utf-8 string)

[cloudwu]

I submit a PR to imgui today, ocornut/imgui#2541 .

We can use AddInputCharacter for full unicode code point later (After the PR merged) .

btw, I found the same issue for WM_CHAR in bgfx https://github.com/bkaradzic/bgfx/blob/master/examples/common/entry/entry_windows.cpp#L843-L861

The _wparam of WM_CHAR is UTF-16, we should handle the surrogate here, because some code point will be encoded into two WM_CHAR messages. There is a similar issue : SFML/SFML#366

[bkaradzic]

You can send PR for entry_windows issue. I don't really know what should be done. :)