#57 Pluggable look-and-feel support

[Frank W. Zammetti]

Implement a system for allowing non-default Swing L&F's to be used in DV. This could come in a number of different forms, but the one I tend to prefer is a list of pre-determined possibilities as part of a new Preferences dialog that they can choose from. Alternatively, a command-line switch for specifying L&F could be added (and these two could coexist as well). We might or might not give the ability to add L&F's generically.