use HKCU for reading/writing prefs in Windows, and fall back to HKLM for read only.
This change uses the HKCU (HKEY_CURRENT_USER) key for making changes to preferences instead of HKLM (HKEY_LOCAL_MACHINE). The reason is that Windows 10 apparently no longer (easily) allows changes from the program to be made to HKLM which means the prefs changes don't stick.
An outstanding issue is that the HKCU entries for Purr Data don't get removed on uninstall. I'm not sure how to do that.