io/system: add ClipboardEvent for sending clipboard updates

Not wired up yet; each platform needs implementing low-level access
first.

Updates gio#31

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

io/system/system.go

@@ -64,6 +64,12 @@ type DestroyEvent struct {
 	Err error
 }
 
+// ClipboardEvent is sent once for each request for the
+// clipboard content.
+type ClipboardEvent struct {
+	Text string
+}
+
 // Insets is the space taken up by
 // system decoration such as translucent
 // system bars and software keyboards.
@@ -115,7 +121,8 @@ func (l Stage) String() string {
 	}
 }
 
-func (FrameEvent) ImplementsEvent()    {}
-func (StageEvent) ImplementsEvent()    {}
-func (*CommandEvent) ImplementsEvent() {}
-func (DestroyEvent) ImplementsEvent()  {}
+func (FrameEvent) ImplementsEvent()     {}
+func (StageEvent) ImplementsEvent()     {}
+func (*CommandEvent) ImplementsEvent()  {}
+func (DestroyEvent) ImplementsEvent()   {}
+func (ClipboardEvent) ImplementsEvent() {}