Added a little clipboard interface

backend.go

@@ -20,6 +20,13 @@ type Backend interface {
 	// and returns a struct representing it that fulfills the Window
 	// interface.
 	NewWindow (width, height int) (window Window, err error)
+
+	// Copy puts data in the clipboard.
+	Copy (data Data)
+
+	// Paste returns the data currently in the clipboard. This method may
+	// return nil.
+	Paste () (data Data)
 }
 
 // BackendFactory represents a function capable of constructing a backend

data.go

@@ -0,0 +1,25 @@
+package tomo
+
+import "io"
+
+// Data represents drag-and-drop, selection, or clipboard data.
+type Data interface {
+	io.Reader
+
+	// 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 {
+	// Type is the first half of the MIME type, and Subtype is the second
+	// half. The separating slash is not included in either. For example,
+	// text/html becomes:
+	// Mime { Type: "text", Subtype: "html" }
+	Type, Subtype string
+}