Add tooltips (hints) and other changes to the prefs window

[Moon-0xff]

This PR supersedes #21 (and #19 precedes #21)

It adds hints(tooltips) for certain options and other changes to prefs.js

[Moon-0xff]

I think we should use tooltips only when:

• The option isn't self-explanatory (when changing the option values there's no apparent effect)
• There's extra information worth explaining
• The option explanation is too lengthy to be the description

[Moon-0xff]

I think it's ready to merge