Add installer option to perform user-level install

The new installer always asks for Administrator permissions even if the user chooses to install in a location to which they have full permissions. This is despite the fact that in Advanced Installer I picked the option 'Admin level install if user in admin, user-level install otherwise'.

I checked this with the Advanced Installer team, and it turns out that the option is somewhat badly named. What it actually does is perform an Admin-level 'All Users' install if it's possible to escalate to Admin, not just if the user is already Admin - this means that on Vista upwards you always get a request to escalate to Admin.

In order to truly perform a user-level install we have to add an extra dialog to the installer to let the user choose whether to install for all users, or just for themselves.