better Input Method Editor (IME) support

Features:

- Allow input methods swap with hotkey (E.g. left ctrl + left shift).
- Over-the-spot pre-editing style, pre-edit data placed over insertion point.
- Restart IME without segmentation fault.

TODO:

- Automatically pickup IME if st started before IME

st.c

@@ -2594,6 +2594,7 @@ draw(void)
 			term.ocx, term.ocy, term.line[term.ocy][term.ocx]);
 	term.ocx = cx, term.ocy = term.c.y;
 	xfinishdraw();
+	xximspot(term.ocx, term.ocy);
 }
 
 void