Made the clipboard API a bit better

This should work better with the X clipboard system better when I
eventually make use of that.

backend.go

@@ -22,11 +22,10 @@ type Backend interface {
 	NewWindow (width, height int) (window Window, err error)
 
 	// Copy puts data into the clipboard.
-	Copy (data Data)
+	Copy (Data)
 
-	// Paste returns the data currently in the clipboard. This method may
+	// Paste returns the data currently in the clipboard.
-	// return nil.
+	Paste (accept []Mime) (Data)
-	Paste () (data Data)
 }
 
 // BackendFactory represents a function capable of constructing a backend

backends/x/x.go

@@ -88,7 +88,7 @@ func (backend *Backend) Copy (data tomo.Data) {
 
 // Paste returns the data currently in the clipboard. This method may
 // return nil. This method is not yet implemented and will do nothing!
-func (backend *Backend) Paste () (data tomo.Data) {
+func (backend *Backend) Paste (accept []tomo.Mime) (data tomo.Data) {
 	backend.assert()
 	// TODO
 	return

data.go

@@ -2,18 +2,9 @@ package tomo
 
 import "io"
 
-// Data represents drag-and-drop, selection, or clipboard data.
+// Data represents arbitrary polymorphic data that can be used for data transfer
-type Data interface {
+// between applications.
-	io.Reader
+type Data map[Mime] io.ReadCloser
-
-	// Mime returns the MIME type of the data, such as text/plain,
-	// text/html, image/png, etc.
-	Mime () (mimeType Mime)
-
-	// Convert attempts to convert the data to another MIME type. If the
-	// data could not be converted, it should return an error.
-	Convert (to Mime) (converted Data, err error)
-}
 
 // Mime represents a MIME type.
 type Mime struct {
@@ -23,3 +14,7 @@ type Mime struct {
 	// Mime { Type: "text", Subtype: "html" }
 	Type, Subtype string
 }
+
+var MimePlain = Mime { "text", "plain" }
+
+var MimeFile = Mime { "text", "uri-list" }

tomo.go

@@ -48,7 +48,7 @@ func Copy (data Data) {
 
 // Paste returns the data currently in the clipboard. This method may
 // return nil.
-func Paste () (data Data) {
+func Paste (accept []Mime) (Data) {
 	if backend == nil { panic("no backend is running") }
-	return backend.Paste()
+	return backend.Paste(accept)
 }