Add built-in updater (or at least update available notification)

1. Add option to enable built-in updater (or at least update notification)

2. Add selector for time period, e. g. every day/week/2 weeks/month

3. Upon launch, if not already done within current time period, check for update(s)

4. Optionally show changelog

5. Download & install update

Note: TeraCopy should _always_ honor ongoing file actions and execute update installations only after _successful_ finishing of those actions - much like Paint.NET's updater does nowadays.