ui/app: replace WindowOptions with WindowOption

Thanks to Larry Clapp for noticing the opportunity for improvement.

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

ui/app/os_macos.go

@@ -266,7 +266,7 @@ func gio_onCreate(view C.CFTypeRef) {
 	})
 }
 
-func createWindow(win *Window, opts *WindowOptions) error {
+func createWindow(win *Window, opts *windowOptions) error {
 	mainWindow.in <- windowAndOptions{win, opts}
 	return <-mainWindow.errs
 }