shiny: add Compose key support for X11

Fixes golang/go#16332

Change-Id: Id72fe556380acca6c0cb20af7bda3bcec3783b4e
Reviewed-on: https://go-review.googlesource.com/24881
Reviewed-by: Nigel Tao <nigeltao@golang.org>
diff --git a/shiny/driver/internal/x11key/x11key.go b/shiny/driver/internal/x11key/x11key.go
index 4884310..5fa39b6 100644
--- a/shiny/driver/internal/x11key/x11key.go
+++ b/shiny/driver/internal/x11key/x11key.go
@@ -78,6 +78,7 @@
 	xkTab        = 0xff09
 	xkReturn     = 0xff0d
 	xkEscape     = 0xff1b
+	xkMultiKey   = 0xff20
 	xkHome       = 0xff50
 	xkLeft       = 0xff51
 	xkUp         = 0xff52
@@ -134,6 +135,7 @@
 	xkEnd:        key.CodeEnd,
 	xkInsert:     key.CodeInsert,
 	xkMenu:       key.CodeRightGUI, // TODO: CodeRightGUI or CodeMenu??
+	xkMultiKey:   key.CodeCompose,
 
 	xkF1:  key.CodeF1,
 	xkF2:  key.CodeF2,