ImmIsIME() doesn't work (always returns TRUE) since Vista.
Use ITfActiveLanguageProfileNotifySink to detect TSF changes,
which are equal to IME changes for us.

Also make sure that IMMultiContext re-loads the IM when keyboard layout
changes, otherwise there's a subtle bug that could happen:
* Run GTK application with non-IME layout (US, for example)
* Focus on an editable widget (GtkEntry, for example)
* IM Context is initialized to use the simple IM
* Switch to an IME layout (such as Korean)
* Start typing
* Since IME module is not loaded yet, keypresses are handled
  by a default MS IME handler
* Once IME commits a character, GDK will get a WM_KEYDOWN,
  which will trigger a GdkKeyEvent, which will be handled by
  an event filter in IM Context, which will finally re-evaluate
  its status and load IME, and only after that GTK will get
  to handle IME by itself - but by that point input would
  already be broken.
To avoid this we can emit a dummy event (with Void keyval),
which will cause IM Context to load the appropriate module
immediately.