cleanup/modernize installers: phase 3

Now that AU is part of the install tree, the following should be done:

-move the AU common code to install/src/windows/common area
-move all code/functions shared by wrappers/installer.dll and au2.0 to this location as well
-remove all duplicate code/functions from wrapper/installer.dll/au2.0 code, and instead link to the new common location.

Once this is all done, further cleanup/modernization of all components will be easier and more maintainable.