Convert .Rproj file extension to associated UTI 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.
Merge pull request #153 from rstudio/bugfix/rproj-spotlight-search

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