Draft: Issue #1382: Let user choose install location on Windows

This patch attempts to provide a solution for issue #1382

According to the documentation, changing DisableDirPage from auto to no will make the installer show the install location whenever it is rerun. It should still default to showing the prior install location, so users can just click Okay without having to re-find their previous installation.

I want to verify that this works with the current build process, then I'll ping Windows contributors for input.

EDIT: Ah right, it won't build here because of the current issue with 32 bit. 😄