app: move system.ClipboardEvent to its own package

API change. Update your code with gofmt rule and goimports:

gofmt -r "system.ClipboardEvent -> clipboard.Event"
goimports

Signed-off-by: Inkeliz <inkeliz@inkeliz.com>
Signed-off-by: Elias Naur <mail@eliasnaur.com>

app/window.go

@@ -195,7 +195,7 @@ func (w *Window) Invalidate() {
 }
 
 // ReadClipboard initiates a read of the clipboard in the form
-// of a system.ClipboardEvent. Multiple reads may be coalesced
+// of a clipboard.Event. Multiple reads may be coalesced
 // to a single event.
 func (w *Window) ReadClipboard() {
 	go w.driverDo(func() {