Add settings window

[rs22]

Browser settings (such as worker count, start page) should be set in a central settings window.

Should we extend the Squeak Preference browser (Tools -> Preferences) or have our own window?

Estimated effort: 150

[MrSerth]

I think it would be fine to have settings in Scamper, because as a user I would not search settings for Scamper in the Squeak Preference browser.