Windows installer improvements from Claus

Windows installer improvements from Claus
- add link to inno setup docs, so readers don't have to guess what
  ghc.iss might be

- add Task section, and associated most Registry actions with separate
  (sub)tasks, so that file associations, default action, and PATH
  setting can be optional

- copy license file into doc directory

- install icon file again, so that DefaultIcon is no longer a dangling
  pointer (#4352)

- only delete ghc_haskell key if empty (in case there were other tools
  using it)

- add versioned GHCi to right-click menu (to allow for multiple tool installs)