Added x clipboard stub

It doesn't do anything yet but boy is it going to

backend.go

@@ -21,7 +21,7 @@ type Backend interface {
 	// interface.
 	NewWindow (width, height int) (window Window, err error)
 
-	// Copy puts data in the clipboard.
+	// Copy puts data into the clipboard.
 	Copy (data Data)
 
 	// Paste returns the data currently in the clipboard. This method may

backends/x/x.go

@@ -79,6 +79,21 @@ func (backend *Backend) Do (callback func ()) {
 	backend.doChannel <- callback
 }
 
+// Copy puts data into the clipboard. This method is not yet implemented and
+// will do nothing!
+func (backend *Backend) Copy (data tomo.Data) {
+	backend.assert()
+	// TODO
+}
+
+// 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) {
+	backend.assert()
+	// TODO
+	return
+}
+
 func (backend *Backend) assert () {
 	if backend == nil { panic("nil backend") }
 }

tomo.go

@@ -24,6 +24,7 @@ func Stop () {
 // Do executes the specified callback within the main thread as soon as
 // possible. This function can be safely called from other threads.
 func Do (callback func ()) {
+	if backend == nil { panic("no backend is running") }
 	backend.Do(callback)
 }
 
@@ -36,6 +37,18 @@ func NewWindow (width, height int) (window Window, err error) {
 		err = errors.New("no backend is running.")
 		return
 	}
-	window, err = backend.NewWindow(width, height)
+	return backend.NewWindow(width, height)
-	return
+}
+
+// Copy puts data into the clipboard.
+func Copy (data Data) {
+	if backend == nil { panic("no backend is running") }
+	backend.Copy(data)
+}
+
+// Paste returns the data currently in the clipboard. This method may
+// return nil.
+func Paste () (data Data) {
+	if backend == nil { panic("no backend is running") }
+	return backend.Paste()
 }