Unicode, Windows: Remove stringapiset.h include (breaks vs2010 and seems unnecessary?). (#2541, #2815)

docs/TODO.txt

@@ -253,6 +253,7 @@ It's mostly a bunch of personal notes, probably incomplete. Feel free to query i
  - log: let user copy any window content to clipboard easily (CTRL+C on windows? while moving it? context menu?). code is commented because it fails with multiple Begin/End pairs.
  - log: obsolete LogButtons() all together.
  - log: LogButtons() options for specifying depth and/or hiding depth slider
+ - log: enabling log ends up pushing and growing vertices buffersbecause we don't distinguish layout vs render clipping
 
  - filters: set a current filter that tree node can automatically query to hide themselves
  - filters: handle wild-cards (with implicit leading/trailing *), reg-exprs

imgui.cpp

@@ -830,7 +830,6 @@ CODE
 #else
 #include <windows.h>
 #endif
-#include <stringapiset.h>   // MultiByteToWideChar, WideCharToMultiByte
 #if defined(WINAPI_FAMILY) && (WINAPI_FAMILY == WINAPI_FAMILY_APP) // UWP doesn't have all Win32 functions
 #define IMGUI_DISABLE_WIN32_DEFAULT_CLIPBOARD_FUNCTIONS
 #define IMGUI_DISABLE_WIN32_DEFAULT_IME_FUNCTIONS