Unicode: full Unicode Support (6 squashed commits) (#2541, #2538)

fix build for WideCharToMultiByte
[3181ff1e] Full Unicode Support
[6c9e73ac] Fix ImTextCountUtf8BytesFromChar and ImTextCharToUtf8, these APIs assume the input is an unicode code point, not UTF-16
[ba85665b] Add AddInputCharacterUTF16 for windows backend to handle WM_CHAR
[fafdcaf0] Use Windows API to convert UTF-16 for ImFileOpen
[dc7d5925] Use windows API to convert UTF-16 for clipboard
5 files changed