Implemented Window.Center() on Mac OS X.

objc_darwin.h

@@ -116,6 +116,7 @@ extern BOOL isCheckboxChecked(id);
 extern void windowSetContentSize(id, intptr_t, intptr_t);
 extern void setProgress(id, intptr_t);
 extern void setAreaSize(id, intptr_t, intptr_t);
+extern void center(id);
 
 /* combobox_darwin.m */
 extern id makeCombobox(BOOL);

sysdata_darwin.go

@@ -412,3 +412,13 @@ func (s *sysData) repaintAll() {
 	}
 	<-ret
 }
+
+func (s *sysData) center() {
+	ret := make(chan struct{})
+	defer close(ret)
+	uitask <- func() {
+		C.center(s.id)
+		ret <- struct{}{}
+	}
+	<-ret
+}

sysdata_darwin.m

@@ -229,3 +229,8 @@ void setAreaSize(id scrollview, intptr_t width, intptr_t height)
 	[area setFrame:NSMakeRect(0, 0, (CGFloat) width, (CGFloat) height)];
 	[area display];			// and redraw
 }
+
+void center(id w)
+{
+	[toNSWindow(w) center];
+}