Input Method Editor (IME) support for text buttons

Original patch by @random (D765) with some minor work done by @campbell
and me.

At this place, I'd like call out a number of people who were involved and
deserve a big "Thank you!":
* At the first place @randon who developed and submitted the patch
* The Blendercn community which helped a lot with testing - espacially
* @yuzukyo, @leon_cheung and @kjym3
* @campbellbarton, @mont29 and @sergey for their help and advises during
* review
* @ton who realized the importance of this early on and asked me for
* reviewing

We are still not finished, as this is only the first part of the
implementaion, but there's more to come!

source/blender/blenloader/intern/readfile.c

@@ -5677,6 +5677,9 @@ static void direct_link_windowmanager(FileData *fd, wmWindowManager *wm)
 		win->eventstate = NULL;
 		win->curswin = NULL;
 		win->tweak = NULL;
+#ifdef WIN32
+		win->ime_data = NULL;
+#endif
 		
 		BLI_listbase_clear(&win->queue);
 		BLI_listbase_clear(&win->handlers);