Unicode: UTF32 support improvements (#2541, #2538, #2815)

- Make ImWchar32 unsigned.
 - Fix Win32 version of ImFileOpen by including windows.h sooner.
 - Make ImGuiIO::AddInputCharacterUTF16() more robust by disallowing illegal
surrogate pairs.
 - Allow pushing higher plane codepoints through ImGuiIO::AddInputCharacter().
 - Minor cleaning up in the high-plane Unicode support.
 - Fix Clang -Wunreachable-code warning
3 files changed