Import Krita-specific NSIS installer scripts
Split the Krita installer scripts from the [shell extension source
tree][1] so they are now tracked with Krita. The shell extension itself
and the scripts related to it will still be managed separately.
[1]: https://github.com/alvinhochun/KritaShellExtension/commits/ce580393b046d03d297bc53cee5d96efa9b25227