Installer

I made the installer with NSIS, because that's what works the best for me ATM.

I kind of want to make a custom installer because crappy antivirus keep flagging NSIS-related stuff as malicious, but it's difficult. I may not end up having one completed.

For some reason, the Krita installer script is also included in this repo. (Oh I think I know why, because the Krita installer shares some code with the shell extension installer, right? Or is that because I am maintaining the scripts?)