lots of installer improvements:

lots of installer improvements:
* use nsDialogs instead of InstallOptions for custom wizard pages, faster & easier to customize
* upgrade to Modern UI 2
* use MultiUser.nsh to support multiple installation modes (all users / current user) in a much more simple way
* use LangFile.nsh for the language files, again less scripting required
* simplified installer structure
* more meaningful variable names
* more comments
* minor fixes

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@24062 a592a061-630c-0410-9148-cb99ea01b6c8