Preferences should have an associated type, refactor needed?

[clintval]

These preferences each have a specific type associated with them, so constant object values are probably not the best way to store this data:

https://github.com/clintval/cvbio/blob/b6965defbcb75cc9b73ac4e66e766f472c025e72/tools/src/com/cvbio/tools/igv/IgvPreferences.scala#L3-L39

All preferences defined here:

https://github.com/igvteam/igv/blob/master/src/main/resources/org/broad/igv/prefs/preferences.tab