UI to choose file types to associate #305

I think many of our users would love to be capable of viewing SAS .sas7bdat files or Stata .dta files on their machine if they do not have either installed BUT would also hate it if the larray-editor would hijack the file association if they have those programs installed. Since I don't know how to detect the presence of those, and it can change in time anyway, the only option I see is have an UI to let users choose which types to associate... But menuinst does not seem to provide a nice Python API we could call into.