Add ImGuiIO::AddInputCharactersUTF8(char* utf8str)

It'll convert the utf8 string to ImWchar's and passes each of them
to AddInputCharacter().

Very handy for SDL2 SDL_TEXTINPUT events, which provide a buffer with an
UTF-8 string.

imgui.h

@@ -705,6 +705,7 @@ struct ImGuiIO
 
     // Function
     IMGUI_API void AddInputCharacter(ImWchar c); // Helper to add a new character into InputCharacters[]
+    IMGUI_API void AddInputCharactersUTF8(const char* utf8chars); // Helper to add new characters into InputCharacters[] from an utf8-string
 
     //------------------------------------------------------------------
     // Output - Retrieve after calling NewFrame(), you can use them to discard inputs or hide them from the rest of your application