introduces use of preference: `'helios.browser.showComment' asSetting` in HLBrowserModel

moves `onbeforeunload` callback from index.html to HLManager class >>setup

introduces 'helios.confirmOnExit' asSettingIfAbsent: true
using settingValue family of conveniences
Merge branch 'feature/preferences' of github.com:amber-smalltalk/helios into feature/preferences

Conflicts:
	src/Helios-Browser.js
	src/Helios-Browser.st
	src/Helios-Core.js
	src/Helios-Core.st