widget: add drag and drop support

This patch adds internal Drag and Drop support to app.Windows.

The new package io/transfer adds the ability to
define draggable and droppable targets, which
are leveraged by the new widget.Draggable type.

The API is generic and could handle future use
cases, such as external Drag and Drop.

Updates gio#153

Signed-off-by: Pierre Curto <pierre.curto@gmail.com>

internal/ops/ops.go

@@ -51,6 +51,9 @@ const (
 	TypePointerInput
 	TypeClipboardRead
 	TypeClipboardWrite
+	TypeSource
+	TypeTarget
+	TypeOffer
 	TypeKeyInput
 	TypeKeyFocus
 	TypeKeySoftKeyboard
@@ -129,6 +132,9 @@ const (
 	TypePointerInputLen     = 1 + 1 + 1*2 + 2*4 + 2*4
 	TypeClipboardReadLen    = 1
 	TypeClipboardWriteLen   = 1
+	TypeSourceLen           = 1
+	TypeTargetLen           = 1
+	TypeOfferLen            = 1
 	TypeKeyInputLen         = 1 + 1
 	TypeKeyFocusLen         = 1 + 1
 	TypeKeySoftKeyboardLen  = 1 + 1
@@ -240,6 +246,12 @@ func Write2(o *Ops, n int, ref1, ref2 interface{}) []byte {
 	return o.data[len(o.data)-n:]
 }
 
+func Write3(o *Ops, n int, ref1, ref2, ref3 interface{}) []byte {
+	o.data = append(o.data, make([]byte, n)...)
+	o.refs = append(o.refs, ref1, ref2, ref3)
+	return o.data[len(o.data)-n:]
+}
+
 func PCFor(o *Ops) PC {
 	return PC{data: len(o.data), refs: len(o.refs)}
 }
@@ -353,6 +365,9 @@ func (t OpType) Size() int {
 		TypePointerInputLen,
 		TypeClipboardReadLen,
 		TypeClipboardWriteLen,
+		TypeSourceLen,
+		TypeTargetLen,
+		TypeOfferLen,
 		TypeKeyInputLen,
 		TypeKeyFocusLen,
 		TypeKeySoftKeyboardLen,
@@ -377,8 +392,10 @@ func (t OpType) NumRefs() int {
 	switch t {
 	case TypeKeyInput, TypeKeyFocus, TypePointerInput, TypeProfile, TypeCall, TypeClipboardRead, TypeClipboardWrite, TypeCursor, TypeSemanticLabel, TypeSemanticDesc:
 		return 1
-	case TypeImage:
+	case TypeImage, TypeSource, TypeTarget:
 		return 2
+	case TypeOffer:
+		return 3
 	default:
 		return 0
 	}
@@ -418,6 +435,12 @@ func (t OpType) String() string {
 		return "ClipboardRead"
 	case TypeClipboardWrite:
 		return "ClipboardWrite"
+	case TypeSource:
+		return "Source"
+	case TypeTarget:
+		return "Target"
+	case TypeOffer:
+		return "Offer"
 	case TypeKeyInput:
 		return "KeyInput"
 	case TypeKeyFocus: