Convert .Rproj file extension to associated UID on 'Open Project'

This fixes a bug wherein spotlight search failed within 'Open Project'... dialogs. If the file extension is '.Rproj', we pass the associated Uniform Type Identifier, rather than the extension itself, so that search functions correctly.

NOTE: I took the UTI from a sample `.Rproj` file with e.g. `mdls devtools.Rproj`, which gave back:

```
kMDItemContentType             = "dyn.ah62d4rv4ge81e6dwr7za"
kMDItemContentTypeTree         = (
    "dyn.ah62d4rv4ge81e6dwr7za",
    "public.data",
    "public.item"
)
```

and so that is how I obtained the ID; however, I think there must be some better method of programmatically getting the UTI, but I haven't been able to figure it out. But we should double-check that it's consistent across platforms.