event/key: add Compose key

Fixes #16332

Change-Id: I4fcd9c4c60fb5c32abaab5b68d3036d536ab2b69
Reviewed-on: https://go-review.googlesource.com/24880
Reviewed-by: Nigel Tao <nigeltao@golang.org>

event/key/code_string.go

@@ -1,4 +1,4 @@
-// generated by stringer -type=Code; DO NOT EDIT
+// Code generated by "stringer -type=Code"; DO NOT EDIT
 
 package key
 
@@ -13,6 +13,7 @@ const (
 	_Code_name_5 = "CodeHelp"
 	_Code_name_6 = "CodeMuteCodeVolumeUpCodeVolumeDown"
 	_Code_name_7 = "CodeLeftControlCodeLeftShiftCodeLeftAltCodeLeftGUICodeRightControlCodeRightShiftCodeRightAltCodeRightGUI"
+	_Code_name_8 = "CodeCompose"
 )
 
 var (
@@ -24,6 +25,7 @@ var (
 	_Code_index_5 = [...]uint8{0, 8}
 	_Code_index_6 = [...]uint8{0, 8, 20, 34}
 	_Code_index_7 = [...]uint8{0, 15, 28, 39, 50, 66, 80, 92, 104}
+	_Code_index_8 = [...]uint8{0, 11}
 )
 
 func (i Code) String() string {
@@ -50,6 +52,8 @@ func (i Code) String() string {
 	case 224 <= i && i <= 231:
 		i -= 224
 		return _Code_name_7[_Code_index_7[i]:_Code_index_7[i+1]]
+	case i == 65536:
+		return _Code_name_8
 	default:
 		return fmt.Sprintf("Code(%d)", i)
 	}

event/key/key.go

@@ -218,6 +218,17 @@ const (
 	CodeRightShift   Code = 229
 	CodeRightAlt     Code = 230
 	CodeRightGUI     Code = 231
+
+	// The following codes are not part of the standard USB HID Usage IDs for
+	// keyboards. See http://www.usb.org/developers/hidpage/Hut1_12v2.pdf
+	//
+	// Usage IDs are uint16s, so these non-standard values start at 0x10000.
+
+	// CodeCompose is the Code for a compose key, sometimes called a multi key,
+	// used to input non-ASCII characters such as ñ being composed of n and ~.
+	//
+	// See https://en.wikipedia.org/wiki/Compose_key
+	CodeCompose Code = 0x10000
 )
 
 // TODO: Given we use runes outside the unicode space, should we provide a