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/windowmanager/WM_api.h

@@ -462,6 +462,10 @@ void        WM_event_ndof_to_quat(const struct wmNDOFMotionData *ndof, float q[4
 float       WM_event_tablet_data(const struct wmEvent *event, int *pen_flip, float tilt[2]);
 bool        WM_event_is_tablet(const struct wmEvent *event);
 
+#ifdef WITH_INPUT_IME
+bool        WM_event_is_ime_switch(const struct wmEvent *event);
+#endif
+
 #ifdef __cplusplus
 }
 #endif