manage LaunchAgent plist file in app via menu item

[progrium]

I'm not sure why I avoided it, but instead of generating a plist file for the user to place at ~/Library/LaunchAgents/com.progrium.Topframe.plist via command line, there should just be a menu item for "Start at login". We'd check if that plist file exists to "checkmark" that menu item. On click, if not checked, it would write that file. If checked, it would delete the file. I know that would remove any edits made to the file, but I don't know of another simple way to do this. Maybe that's why I wanted the user to manage it in the first place. However, I think the convenience for the common case is worth it.