Register .snpt file association

[Remi05]

It would be convenient to automatically register Outlines as the default app to open .snpt (Snapshot) files. This would eliminate the need for users to select Outlines when trying to open these files for the first time.

Useful links:

• https://docs.microsoft.com/en-us/windows/win32/shell/how-to-register-a-file-type-for-a-new-application

[Remi05]

Added with PR #99.