Blender Git Loki

Windows installer and Path changes, fixing various issues:

* Windows installer not working for non-admin users and multiple users
* Addon scripts not installing next to user configuration
* Portable install not being taken into account in all places

The main problem was the windows installer was installing system scripts in
AppData next to the user configuration directory, which is not shared between
users. Now these are installed in ProgramFiles, and only addon scripts added
by the users go to AppData.

On all platforms, addon scripts were sometimes getting installed between
system scripts, because the scripts folder in the executable directory was
given precedence over the user configuration folder, that is no longer done
now. So addons now behave like user configuration, they are preserved even
if you download a newer build of the same blender version.

If you have an installation of 2.57 on windows, the addon install location
will not change until we do the version bump to 2.58, to avoid conflicts with
the existing the installed 2.57 version.

The old behavior of giving precedence to the local folder was done to support
portable install, where all configuration is written to the local folder. This
is now implemented differently: if and only if a "config" folder exists in the
local folder, portable install will be assumed, and files will only be written
to that local folder.